Understanding Light: From Ancient Theories to Modern Physics
The Nature of Light
The nature of light has always interested humankind. Scientists, philosophers, and thinkers have attempted to answer seemingly simple questions, such as “What is light made of?” since the oldest civilizations. Science, particularly physics, has no simple answers to these questions. For example, to answer that light consists of photons does not respond unless one agrees on the meaning of the word photon. A photon is not an object that belongs to our ordinary experience, like a ball, nor is it like anything else known to all. The photon concept was developed to explain diverse phenomena of light, some apparently contradictory, from simple assumptions. You could say that this concept arose from a “survival” transformation of our ideas to confront new luminous phenomena. Or, put another way, the concept of photon is the result of the evolution of our ideas about the nature of light, and it can explain, without contradiction, all known light phenomena.
A photon is unlike anything of our common experience. It is not like a ball, or a marble, nor like sound waves, although in some aspects, its events are like the behavior of material particles and in others, like waves.
However, the representation of an idea in the form of images of known objects is not essential to science; the important thing is that with that idea, we can describe phenomena. The ideas about light have depended, then, on the luminous phenomena that have attempted to explain them. When only the most common luminous phenomena were known, ideas about the nature of light were very simple, but as more complex phenomena of light became known, these ideas changed to accommodate the known and new phenomena in the same theory. For the same reasons, it is possible to understand modern ideas about light without knowing the luminous phenomena that gave rise to them. This work develops ideas about light from the discussion of some of the most important phenomena and experiments on it, from the early Greek ideas about light to the contemporary, in which atomic-scale phenomena are involved, and the concept of the photon.
I. Rays of Touch
In pre-Greek civilization, ideas about the nature of light were full of mystery. Also, they never stayed for long: they disappeared and returned in slightly different forms. The ancient Greeks produced the first useful ideas about light, and possibly because of this, they were argued for centuries. The Greeks did not clearly distinguish light from sight, and both based their ideas on the assumption of tactile visual rays attributed to Pythagoras. Under this scenario, the eye emits straight, infinitely thin rays that, when interrupted by objects, produce the sensation of seeing. These rays should be straight to explain the rectilinear propagation of light, that is, to explain the fact that we see through a straw only if it is straight (Figure 1).
Figure 1. The rectilinear propagation of light can be demonstrated with this simple experiment. The candle is seen through the straw only if it is straight.
Perception by means of these visual rays would then be analogous to the tactile sense when using arms and hands to discern the shape and size of objects. A large object separates the touch rays more than a small object, and this increased angular separation of the rays would produce in the mind the larger sense of the larger object (Figure 2).
Figure 2. The hypothesis of visual rays of Pythagoras assumed that these were cast by the eyes and, being interrupted by the objects, produced the sensation of seeing. The size of the objects was perceived by the separation of the interrupted rays.
The hypothesis also explained the decrease in the apparent size of an object moving away, as the touch rays interrupted by the object form an angle less and less, until reduced to zero, as the object moves away from the observer. This would explain why parallel lines appear to converge indefinitely at a point, later renamed the “vanishing point” by Renaissance artists (Figure 3). Moreover, under this assumption, the apparent size reduction would be in the same proportion as the increase in distance, i.e., if the distance increased twice, the apparent size would also decrease twice. As this is precisely what happens to the apparent size with increasing distance, the hypothesis of the touch rays was given backing by experience, at least for this experience.
Figure 3. The visual rays hypothesis explained the apparent decrease in the size of an object as it moves away; the angular separation of interrupted beams is reduced with increasing distance between the eye and the object. Thus, parallel lines appear to converge indefinitely to a point on the horizon that later became known as the “vanishing point”.
The hypothesis of tactile rays was useful because it mathematically related the apparent size and distance, and could be used in many practical activities such as design and layout of works of architecture or engineering. More important to the Greeks was its application to astronomy to calculate distances and sizes of celestial bodies, for example, to calculate the diameter of the Sun. These applications to astronomy enabled them to gain insight into the size of the universe supported by observations and, above all, supported by geometry, which was the perfect science of Greek culture. All this must have contributed to the hypothesis that visual touch rays were accepted even by Euclid himself, the founder of geometry, and that it would last about 1,500 years without being seriously questioned.
II. Light Rays
The theory of Pythagorean touch rays prevailed for over 1,500 years. This long life of such a naive theory was probably due to the lack of rigorous experimentation that put it to the test because it really does not stand up under experimental challenge. The hypothesis of tactile rays emanating from the eye was suddenly demolished by an eccentric Arab scientist named Abu Ali al-Hasan ibn al-Haytham, who was born in Basra, Iraq, around 965 AD, died in Cairo, Egypt, in the year 1039, and later known simply as Alhazen. This unique character arrived in Egypt in 996 AD, hired by the caliph Al-Hakim to control the flooding of the Nile, which Alhazen publicly boasted of being able to do without much difficulty. Unable, however, to fulfill his promise and fearing the wrath of the caliph, Alhazen feigned madness until the death of the caliph in the year 1021, to avoid the death penalty for his failure.
Despite his problems with the river Nile and the Caliph Al-Hakim, Alhazen could do important work in optics research, or the science of light. In his major work, entitled Kitab al-Manazir in Arabic and translated into Latin as Opticae Thesaurus, Alhazen demonstrates that vision cannot be due to rays emanating from the eye to the object, but from the object to the eye. Thus, he clearly separated the light from the sense of sight. A simple experiment that demonstrates this is well known for producing fire by means of a lens focusing the image of the Sun on a piece of paper (Figure 4). If tactile rays existed, the image of the Sun on the paper would be formed by visual rays that would otherwise have been reflected in the paper, passed through the lens, and reached the Sun. The paper, therefore, should not catch fire if one closed their eyes or looked the other way while maintaining the focused image. But the paper itself becomes inflamed if the magnifier is held at a proper distance from the paper, whatever we do, so that the image is formed by something that comes from the Sun, and not from our eyes to the paper.
Figure 4. Important optical phenomena in the 13th century. The rectilinear propagation of light, the mirror image reflection, refraction of light in water, the heating power of the Sun’s rays concentrated by a lens, and the appearance of the rainbow.
Another experiment that also demonstrates the existence of light regardless of the sense of sight is the formation of images in the simple tool called “camera obscura” (Figure 5). This instrument uses a small hole to produce an image of an external object on a screen placed in a dark room or in a simple cardboard box. The image you see is always inverted, and this cannot be explained by the touch rays, because the object could be seen on the screen from inside the chamber only if the rays were reflected on the screen and came out through the hole. In this way, we would perceive the object as if we saw it directly, that is, we would see it erect and not inverted. However, if we assume that each point of the external object emits straight rays in all directions, those starting from a point at the top of the object passing through the hole would produce a small bright point of the image at the bottom of the screen. The full image would be reversed, and this is precisely what is observed.
Figure 5. The pinhole camera produces inverted images on a screen of objects located in front of its pupil. This shows that the hypothesis of visual rays is false.
Drawing on these and other experiments, Alhazen removed forever from the science of light the Pythagorean hypothesis of touch rays, but we still use expressions like “a look” that remind us of it.
III. Geometric Optics
The assumption that each point of a luminous or illuminated object emits straight rays of light in all directions is the main hypothesis of a theory of light extraordinarily fruitful to date; it is called geometrical optics (Figure 6). The name is because, in this theory, the nature of light rays is not questioned, it is not even important. The purpose of the theory is only to partly understand, or predict, what happens to the rays emitted by objects when they are intercepted by various opaque objects, as in the darkroom, or diverted from their straight path in ways that we shall soon see. As this is only necessary to apply knowledge of geometry to each problem, the name of the theory is geometrical optics.
Figure 6. The basic hypothesis of optics after Alhazen. Each point of a luminous object emits light rays straight in all directions.
Drawing on an outline, some simple straight rays are readily seen as shadow regions caused by an opaque body illuminated by luminous bodies. These regions are called, generally, the “geometric shadow” of the body. For example, a sphere illuminated by a single point of light produces a single cone of darkness, called the umbra, where not a ray emitted by the bright spot reaches (Figure 7 (a)). This cone is formed by the tangents to the sphere from the point of light. Outside, light reaches everywhere. But if the opaque sphere is illuminated by a luminous sphere, the cone of darkness, also now limited by the total external tangents to the two areas, there is an area only partly obscured where the light coming from parts of the luminous sphere reaches. This area is called the penumbra (almost shadow) and is limited by the umbra and the cone formed by the interior tangents to the spheres. These zones are clearly observed during lunar eclipses. The Moon takes on a coppery-red color when in the penumbra region and almost completely obscures in the umbra region (Figure 7 (b)).
Figure 7. Geometrical optics explains how the shadow is cast by an opaque body. This region is called the geometric shadow. In figure (a) is the cone formed by the tangent of the sphere. This zone does not reach any ray of light and is called “umbra”. In figure (b) the umbra is the cone formed by the external tangents to the two areas, outside it is an area where light reaches, but only in some parts of the luminous object. This region, called penumbra, is included between the umbra and the inner cone of tangents to the two areas.
The camera obscura is an interesting example of the application of geometrical optics. If in a diagram like Figure 8 we plot the images of the same object placed at different distances from the camera, we find easily that the image size decreases by the same proportion as the distance increases. That is, the relationship of image size with distance is the same as in the case of apparent size in the tactile ray theory. This suggests that the eye functions as a camera obscura. The camera hole is the pupil in the eye, that little black circle placed at the center of the iris. The cavity formed by the eyeball is not empty as in the darkroom, but full of a semi-transparent gelatinous substance called “vitreous humor” that allows light to pass without difficulty. The image of an object is formed at the bottom of the eye on a fine nerve tissue sensitive to light, called the retina, which transmits to the brain by a large number of nerve fibers that come together in a single nerve called the optic nerve. The apparent size of an object depends on the size of the image that forms on the retina. If the object moves away, its image size is reduced and it appears smaller. Thus, the parallel lines that depart from our eyes seem to converge at a distant point on the horizon, called by the artists the ‘vanishing point’ (Figure 3). The variation of the apparent size of objects with distance is the basis of the art of representing objects on a surface as they appear in sight, that is, it is the basis of perspective.
Figure 8. The eye functions like a camera obscura. Light rays passing through the pupil form an image (inverted) of the object on the retina. This, which is in the back of the eye, transmits to the brain via the optic nerve.
There is an interesting problem with the model of the eye as a camera obscura. While on the retina the images are inverted, objects are perceived upright, i.e., standing. This rearrangement is done by the brain and can be shown with the single experiment shown in Figure 9. The eye is illuminated by light passing through a small hole drilled with a pin on a cardboard card that is placed in front of the pupil and about 10 or 15 cm from it. Interposing one end of a chopstick between the bottom of the hole and the pupil, about 2 or 3 inches away, it casts its shadow toward the bottom of the eye. The shadow, however, will appear precisely at the top of the eye, as if the stick had been brought by the top and not the bottom of the hole. This shows that optical impressions made on the retina, images or shadows, are reinvested by the optic nerves and the brain so that the bottom is seen above, left to right, and vice versa. Moreover, the images we see are like those of a camera, but of much higher quality.
Figure 9. Experiment to demonstrate the reinvestment of images by the brain. The shadow of the object projected to the back of the eye is perceived as appearing above, not below the hole on the card.
IV. Reflection and Refraction of Light
The assumption of straight rays of light is not the only hypothesis of geometrical optics. To explain the phenomenon of reflection of light (Figure 4), it is necessary to assume that the direction of the light rays changes in some circumstances. A mirror image looks as if the object were behind it, and not against it. Geometrical optics explains this familiar phenomenon by assuming that the light rays change direction to reach the mirror. The precise way in which this change occurs is known as the law of reflection of light. It is a very simple law: the incident and reflected rays make equal angles with the mirror or the perpendicular to the mirror, which is usually how these angles are measured (Figure 10). This law, incidentally, can also be deduced by applying the law of change of apparent size with distance to explain the apparent sizes of an object and its image in a plane mirror. Or, put another way, if we see our image in a plane mirror the size that we see is because the incident and reflected rays make equal angles with the mirror.
Figure 10. The law of reflection of light: the angle of incidence, i, and reflection, r, of a light beam on a surface are equal, i.e., i = r. The law of refraction of light: the sine of the angle of incidence, sin i, and the sine of the angle of refraction, sin r’, of a light beam through the surface separating two transparent media are in the same proportion for any value of angle i, i.e., sin i / sin r’ = n. If light passes from air to water, sin i / sin r’ = 4 / 3.
A body partially submerged in water looks crooked, as if it bends upon entering the water. This phenomenon is called refraction. In addition to water, it is observed in many other transparent media, such as glass, called refractile. It was one of the optical problems still unresolved in the thirteenth century (Figure 4). Refraction phenomena are incorporated into geometric optics by simply assuming that light rays change direction not only to reflect but also to move from a refracting medium to another, e.g., from water to air, water to glass, or glass to air. A simple experiment that demonstrates this change of direction is shown in Figure 11. A small coin at the bottom of an empty cup is barely concealed by the edge of the cup in Figure 11 (a). Slowly filling the cup with water, the coin is gradually revealed until fully observed, as in Figure 11 (b). The light rays from the currency reaching the eye because they are refracted at the water surface are shown in this figure; the currency is seen in the direction of these rays. The experiment also shows that the refracted rays are closer to the surface in the less dense medium, the air in Figure 11 (b).
Figure 11. An experiment to demonstrate light refraction. In (a) the coin is just hidden by a side of the cup. In (b) the coin is revealed by slowly filling the cup with water. Light rays change direction when passing from water to air.
Exactly how the direction of the rays changes in refraction, i.e., the law of refraction, is not as simple as the law of reflection. Perhaps because of this, although the phenomenon of refraction was known since antiquity, the law of refraction was not discovered until the fifteenth century by the Dutch astronomer Willebrord Snell, who, inexplicably, did not announce it, describing it only in his personal research notes. The law of refraction was released by Descartes in 1627 but is universally known as Snell’s law. It relates the angles of light rays from the perpendicular to the refracting surface, but to the sines of those angles. In mathematical symbols, it is expressed as sin(i) / sin(r’) = constant = n, i.e., the ratio of the sines of the angles of incidence i and refraction r’ takes the same value for all possible values of these angles. For example, if the rays pass from air to water, the constant n, called the refractive index, is 4 / 3, and we have sin(i) / sin(r’) = 4 / 3.
The law of refraction of light can also be deduced by applying the law of change of apparent size with distance. Figure 12 shows a simple experiment to do this. Two small coins are put into two cups, one empty and one partially filled with water. Observing them from above at the same height, the currency plunged into the water appears larger due to the refraction of light; the rays emitted open more when passing through the water surface and reach the eye as having been issued by a nearer coin. From the apparent sizes of the two currencies, the angles formed by the rays from the perpendicular to the surface are deducted; the refracted ray depends on the height of the filling of the cup. The sines of these angles are obtained from a table of values, and dividing the larger by the smaller, it is found that their ratio is always 4 / 3, the refractive index of water, regardless of the filling height of the cup.
Figure 12. An experiment to verify the law of refraction. The currency plunged into the water looks bigger because the rays from it open when they leave for the air and seem to come from a closer coin. Relating the apparent sizes with the angles of the rays yields the law of refraction, or Snell’s law.
The hypothesis of light rays and the laws of reflection and refraction of light are the foundation of geometrical optics. With them, you can predict the course that the light rays will take when reaching lenses or mirrors. For example, in Figure 13, the rays coming from a light to the lens of a common magnifying glass are divergent but are converging upon getting through due to refractions that occur on the surfaces of the glass. After reaching the convergence point, the rays are once again diverging, so if we see them from a place farther away, we perceive them as arising at the point of convergence, i.e., as if the object were transported to there. It is said that at this point, a real image of the object is formed. The laws of refraction allow us to calculate the exact spot where the image is formed. Looking with another magnifying glass in that place shows the magnified image of the object. That is essentially how a telescope works (Figure 14). This instrument uses two lenses of the type called convergent, similar to a magnifying glass, that is thicker in the middle than at the edges. The first one—called the objective—produces a real image of a distant object, like the Moon, at a point behind and close to the lens. The second lens of the telescope, called the eyepiece, is used as a common magnifying glass to amplify and observe this image (Figure 14).
Figure 13. A lens intercepts divergent rays emitted by a light and brings them together at another point. The rays seem to come out of this meeting place. It is said that here a real image of the spot is formed. Figure 14. A telescope consists of a single lens, called the objective, which forms close to it a real image of a distant object, and a magnifying glass, called the eyepiece, with which this image is examined.
Summarizing the above, geometrical optics consists of a hypothesis, that of the straight rays of light, two laws derived from experience, reflection and refraction of light, and a mathematical science, geometry, with which it can be applied to optical problems methodically. Geometrical optics has been extremely successful because it is based on laws that are met with precision and on as complete a science as geometry, but part of its success is a result of its main hypothesis. That is, although it has not even attempted to clarify how light rays are made, they should be made of something that spreads like the rays; otherwise, the theory would not have been so successful.
Isaac Newton assumed that light rays are composed of extremely tiny particles that bodies shed light at high speed, and penetrating the eye and impinging on the retina stimulates vision. Newton supported these ideas on the phenomenon of the rectilinear propagation of light, contributing that only if composed of independent particles could one imagine that the light beams could be separated from each other through a straw, as in Figure 1, or a convergent lens, as in Figure 13. Another important argument that Newton used in support of this idea was that no light turns around opaque objects, or that the geometric shadow of a body is bounded by straight lines, as in Figure 7. This argument was mainly wielded against the ideas of Descartes, who supposed that light was a “kind of pressure” spread around the bodies of light reaching the eye to stimulate vision. But, Newton argued, a pressure zone would not have a reason not to spread around the bodies and into the geometric shadow, i.e., if the light was caused by those “pressure points,” it should also be seen in the geometric shadows of opaque objects.
Newton’s ideas also lead to important conclusions when applied to the refraction of light. Figure 15 attempts to explain the refraction studying the movement of a tennis ball. Because the ball’s speed is different in water than in air, the direction of motion switches upon traversing the surface, i.e., it is refracted. And it can be shown that if the water speed is lower than in the air, the angle of refraction r’ is greater than the incidence i, as shown in this figure. But in the refraction of light, just the opposite occurs; the angle of refraction is less than the incidence in passing from air to water, or upon passing to any denser medium, e.g., glass. It is thus an inevitable conclusion that, should it be composed of particles, light would be faster in denser media. In particular, it should be faster in any transparent medium than in a vacuum. In the time of Newton (1642-1727), it was only possible to measure the speed of light by astronomical means and in no way in a laboratory, as would have been necessary to measure it in water or glass, and compare this with the familiar value for a vacuum. In this way, because it was not possible to penetrate into the knowledge of the nature of light rays, for many years.
Figure 15. The speed of a tennis ball drops, and the direction of motion is closer to the surface upon entering the water. Light, however, when entering the water, moves away from the surface. It follows that if the lamp is made of particles, they would move faster in water than in air.
V. The Diffraction of Light
In Italy—possibly while Newton developed his famous Opticks or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light—an Italian Jesuit, Francesco Grimaldi (1618-1663), physicist and astronomer, who in 1651 gave the names that so far remain to the features of the visible side of the Moon, discovered an important optical phenomenon he himself called diffraction of light. This phenomenon occurs whenever a fraction of the light emitted by a source is separated by interposing an opaque object, and this is what gives rise to its name: division into fractions.
Diffraction can be observed by creating, right in front of an eye, a very narrow slot cut in an opaque sheet, or rather, a groove formed by the edges of two razor blades stuck on a slot with durex cut into a wider strip of cardboard (Figure 16). Looking solely with this eye at a distant light, for example, the flame of a candle placed a few feet away, one would expect to perceive the image of the flame as in Figure 17 (a); however, if the slot is sufficiently narrow, one will perceive multiple images, as in Figure 17 (b). This, of course, is not what we would expect according to geometrical optics. Figure 18 (a) shows the geometric regions of light and shade produced by a slot. If we marked with rays the right eye at the origin of these regions, the light would illuminate the region into the eye and form an image, and only one, of the flame of the candle, which is what we see with a wide slot (Figure 17 (a)). Multiple images seen with the thin slit indicate that, in passing through the slot, the light forms several regions of illumination on both sides of a bright central region that corresponds roughly to the geometric region of illumination. The eye forms images with the rays it receives from each of these regions, which are perceived as in Figure 17 (b).
Figure 16 (a). A thin slit to observe the phenomenon of diffraction of light is built by setting with durex two razors, edge to edge, on a wider groove cut into a strip of cardboard. Before attaching the leaves with durex, the edges are kept apart by the thickness of a strip of paper. (b) The diffraction slot finished. Figure 17. The image of the flame of a candle according to the eye. (a) Through a wide slot, (b) through a thin slit, and diffraction. Figure 18. The areas of light and shade produced by a thin slit. (a) According to geometrical optics. (b) As noted in a diffraction slot.
The phenomenon of diffraction of light and the like are seen most clearly in a dark room and if instead of the candle flame as a light source we use a single bright point. You can get one easily by passing light from the flame of a candle through a small hole drilled in a thick cardboard, preferably black, as shown in Figure 19. Looking at the candle light passing through the hole through the diffraction slot positioned just in front of the eye, there is a set of bright bands of decreasing intensity from the more intense at the center, which is called the diffraction pattern of a slot (Figure 19).
Figure 19. Arrangement to observe the diffraction of a light beam formed by passing light from the flame of a candle through a small hole drilled in a cardboard. Above are shown the diffraction patterns observed with a single slot and a double slot (Figure 21).
The diffraction pattern of a slot seems to deny the rectilinear propagation of light. If we think of light rays, whatever their nature, the images appear to the side as deflected rays coming from the direction of the central rays, i.e., rays that would have bent their course upon passing the edges of the leaves and penetrated into the geometric shadow. The phenomenon of diffraction of light, therefore, contradicts the assumption of straight rays, i.e., it precludes the assumption of rectilinear propagation of light. It seems that light, after all, can turn around opaque objects.
If we think of light rays as consisting of particles, or corpuscles, the phenomenon of diffraction of light also leads to very interesting consequences. We could, for example, imagine a very simple experiment to measure the “size” of such particles; light would pass, just like that from a candle, through more and more narrow slots until one barely allows transmission. The diameter of the “light particles” would be just over the width of this slot. However, watching the flame of a candle through diffraction grooves of different widths, or with a narrow slot of varying width as that of Figure 20, we find that all produce multiple images, i.e., it is found that it is not possible to find a slot that “only permits the passage of light”; to achieve this, it is necessary to close the slot completely. “Particles” according to Newton that compose the light rays seem, therefore, to lack defined dimensions since the light passes through the narrower slots. This surprising result does not show, however, that light is composed of particles; it only shows that, if it were, the tiny particles would not be like rigid balls, marbles, or have defined dimensions.
Figure 20. A diffraction slot of variable width. The edges of the sheet are contacted at one end and separated at the other by the thickness of a piece of paper before securing them with durex to the strip of cardboard.
VI. Wave Optics
The diffraction of light began to think to many scientists, including Grimaldi, its discoverer. The assumption of straight rays bright faces, indeed, serious logical problems to explain this phenomenon. How do you go around the edge rays of the slot and invade the geometric shadow? This is just one of the main questions that arise.
Another problem is the phenomenon of diffraction in the ray theory is the multiplicity of images. Why are several luminous images? Or why the rays transmitted through the slot would prefer to spread in some directions and refusing to do so in others? (Figure 17 (b)).
These problems can be solved by means of ray theory. Their resolution is necessary to completely abandon the hypothesis that light consists of straight rays. The Cartesian idea of the “sort of pressure” that propagates in a medium surrounding the light source explained that the light around obstacles, as this “kind of pressure” would pass through the slot and would spread in the middle that surrounds it, particularly where it is behind her. But to explain the multiple images necessary to assume something else.
This “something more” suggests another optical phenomenon observed by passing light from a source by two adjacent narrow slits. This experiment, known as the double slit, was devised by the English physicist Thomas Young in 1815 and can be done easily with the same elements used to observe diffraction (Figure 16). The double slot is constructed by dividing longitudinally with fine copper wire or silk threadA simple slot with durex are constructed by fixing two pieces of razor edge to edge, on a much wider groove cut into a strip of cardboard disk (Figure 21 (a)). Looking with one eye through the double slit of a candle flame is about 5 meters distant, with the slit placed just in front of the eye open, there is a pattern of multiple images as in Figure 21 (b). In this pattern, as expected, appear side diffraction images of each of the two slots, these are the most distant from the center. But closer to the center, where they form the central band of the diffraction pattern of a slot, now appear brighter and more detailed images than those of diffraction. These multiple images are called “interference images” because they are obtained only if the light coming from a similar site that interferes with light from another site, in this experiment these different sites are the two slots.
Figure 21. (a) A double slit to observe the phenomenon of light interference constructed by dividing a thin slit lengthwise through a thin copper wire or a fine silk thread. Figure 21. (b) interference images of the flame of a candle, center, and diffraction, lateral, resulting from the double slot.
If the experiment is done using, instead of the candle as a bright point in Figure 19, there are several bright razor sharp images of interference of the spot. If the two slots and the wire that separates them are the same width, there are three images in much the same intensity.
Interference phenomena are typical of so-called half wave motion as air or water. A common example is the wave motion that occurs in an area of calm water by throwing a small object in it (Figure 22). The impact of object produces a smallNa deformation composed by a depression and elevation of the surface, which increases in diameter to spread around a circular wave. Fluctuations in water after the impact site produced by other similar waves that follow the first at equal intervals of distance and time. If there are circular waves like this in two spots near the middle, in some places add their effects and produce a larger deformation of the surface because the depressions and elevations of the two waves coincide, while in others the effects are canceled because the depression of one wave coincides with the elevation of the other. In the first sites say that the waves interfere constructively and in the latter, which interfere destructively. These areas of interference can be seen in a diagram like Figure 23. In this, the positions of the waves at a certain instant are represented by concentric circles centered on their original sites S1 and S2. Looking at the diagram at an oblique angle from the opposite end of S1 and S2, there are some darker regions that alternate with more clear, the former are the destructive interference, or negative, of circular waves.
Figure 22. Circular waves formed by dropping a small object in calm water.
Figure 23. Young diagram to observe the areas of constructive interference of circular waves. Viewing the diagram at an oblique angle from the opposite end of these centers are areas that appear darker.
This example suggests that images of interference observed in the double slit experiment result from the constructive interference of wave motions that arise in the two slots. Which is exactly what moves and what means it spreads is not important for now and was not important in the nature of geometrical optics rays to describe many optical phenomena. We think, if we want, what moves are areas that “kind of pressure” Cartesian, which propagate as waves from each slot by adding their effects in places where there are images of interference. This would explain the light to turn around the edges of the groove, and the emergence of multiple images of interference, without requiring clear yet what is this “kind of pressure.” We could even call them “zones of environmental disturbance,” or just areas of “disturbance.”
Multiple images of single-slit diffraction are explained very well, with these ideas of light as a wave phenomenon. We imagine now that the slot is composed of a large number of contiguous slots and more narrow, and that each produces waves of disturbance of the environment at the same pace as others. Combining the effects of these waves at each point behind the slot, or subtraction, adding them as having the same or opposite directions to get to that point, there are areas of higher and lower disturbance that correspond exactly with the light and shade observed visually or with a photograph. It is therefore also the interference of waves, although of many waves, causing the diffraction pattern, and “something more” we sought to explain the wave motion of the disturbed areas of the medium.
Wave optics, or wave theory of light, born of the analogies such as these between optical phenomena and phenomena typical of wave movements known as the wave in liquids, or even known of the acoustic waves that produce the sound. A simple wave motion can be seen from a garden hose attached to a wall at one end and move rhythmically up and down the loose end while he held taut (Figure 24). Each oscillation occurs in the hose hump moving towards the opposite end followed by other similar humps at equal intervals of distance and time, as happens with circular ripples in the water. These waves are called for elastic waves propagating in an elastic medium such as rubber hose.
Figure 24. Elastic waves in a garden hose.
The time it takes to generate a full hump is called period and equals the time needed to run the oscillation paafull ón loose end through the hose. The horizontal distance that occupies a full hump is called wavelength. As each hump moves one wavelength during one period, the forward speed of the wave equals the wavelength divided by period, for example, if each hump has a horizontal length of 0.6 m, and time of each oscillation of the loose end of the hose is 2 s, the wavelength is 0.6 m, the period is 2 s and forward speed of the wave is 0.6 m (2s) = 0.3 m / s . The number of humps that occur each second is equal to the number of oscillations to occur, every second, the loose end of the hose. This amount is called the wave frequency and can be obtained by dividing the unity of the period, in the above example, the frequency is equal to 1 / (2s) = 0.5 / s, or 0.5 Hz (ie Hz). It is easily verified that the forward speed of the wave is also obtained by multiplying the wavelength and frequency, in this example, we have: speed = 0.6 m X 0.5 Hz = 0.3 m / s.
Sound is a wave motion propagating in air and other materials. For example, ringing a bell its vibrations produce, in turn, areas of compression and expansion of surrounding air. These zones are spread in a similar manner to that of circular water waves, but as they can in all directions form a spherical wave (Figure 25). The distance between two consecutive zones of compression or expansion of air is the wavelength and frequency of vibration of the source is also the frequency of the wave. These waves are called, in general. sound waves or noise. Its speed of propagation in air is 330 m / s approximately. On reaching the ear vibrate the eardrum and if the vibration frequency is between about 20 Hz to about 16 000 Hz cause the sensation of sound. The wavelength is between (330 m / s) / (20 Hz) = 16.5 ft (330m / s) / (16 000 Hz) = 0,021 m = 2.1 cm.
Figure 25. Spherical waves of compression and expansion of air caused by the vibrations of a bell. They travel at the speed of sound, 330 m / s.
All wave movements produce similar phenomena. For example, the sound is reflected in solid walls in a manner analogous to the reflection of light in mirrors, this is called echo. The sound goes around obstacles in a manner analogous to the diffraction of light, why can hear conversations around the corner, and one room to another if a door is open. The sound also produces interference phenomena. When two musical notes sound very similar modulations are easily heard in intensity, throbbing calls that result from the interference of waves which produce notes.
These analogies between optical and acoustic phenomena were demonstrated by the experiments with grooves made by Thomas Young around 1815 and gave great strength to the hipótesis that light, like sound is a wave phenomenon resulting from spherical waves that occur at each point of the luminous bodies and propagating in transparent media like air, water, glass or a vacuum. This is the fundamental hypothesis of wave optics and it is possible to understand the unexplained optical phenomena ray theory, and it is even necessary in many cases specify the nature of light waves, ie without having to specify the property environment is disturbed and that propagates in the form of light waves. For example, one can calculate the angles of deflection of light diffracted by a groove knowing only the width of the slot and the wavelength of light, without it being necessary to clarify the ownership of the means which is light waves. Thus, the deflection angle of the light side as the first image is calculated using the formula Æ = 38.2 (L / a) “L” being the wavelength of light and “”The width of the slot. Of course, you can calculate the wavelength of diffracted light and the width of the slot using the above formula in the form L = (Æ / 38.2) a. For example, if the slot width is 0.001cm and the angle of deflection of light forming the first image is de1.9 º lateral, the wavelength of light is L = (1.9 ° / 38.2) X 0,001 = 0.00005 cm. From Thomas Young similarly measured the wavelength of light of each color of the rainbow and found that they are different. The red light, for example, is approximately 0.000075 cm, that of the yellow is about 0.000060 cm and the violet light of 0.000040 cm. between 100 000 and 1 000 000 times smaller than the wavelengths of the sound waves! The frequency of light waves also could be calculated easily because the speed of light, 300 000 km / s = 30 000 000 000 cm / s, was already known. For the yellow light frequency is found to be 30 000 000 000 cm / s) / (0.000,060 cm) = 50 000 000 000 000 Hz This frequency is thousands of millions of times greater than that of sound waves. Quantities very, very small and very, very great made their appearance in physics.
The experiments of Thomas Young gave great strength to the hypothesis of the light wave but were not its origin. Images of the light wave emerged simultaneously with Newton’s corpuscular quite possibly inspired by Cartesian ideas of the “sort of pressure,” propagated in half. Its main proponent was the Dutchman physical, Newton’s contemporary, Hans Christian Huygens, about 1670. However, Huygens spent his wave theory of light primarily to explain problems of reflection and refraction of light as the so-called atmospheric refraction phenomena, for example, the appearance of mirages, the twinkling of the stars or the apparent deformation of the disc of the moon or the sun when near the horizon.
Huygens explained these phenomena from the main wave hypothesis that a light produces spherical waves and that they stimulate the hearing only if the gaze is directed along the wave radio esfEric to reach the eye, that is, only if the light is directed toward the bright spot. This second part of the hypothesis is necessary to include the rectilinear propagation of light (Figure 26). In many phenomena waves change shape for different reasons and are no longer spherical, so can not speak of the radius of the sphere. It is assumed that the wave then stimulates the hearing if the gaze is directed along the perpendicular to the zone of disturbance that is in contact with the eye (Figure 27). This does not change the main hypothesis because if the wave is spherical, the radius perpendicular and have the same direction.
Figure 26. The main hypothesis of wave optics is that each point of light produces spherical waves. Figure 27. If the gaze is directed perpendicular to the disturbed areas stimulates the sense of sight. Spherical waves in this direction coincides with a radius of the sphere.
To explain the phenomena of atmospheric refraction are diagrams in which the spherical waves are represented by concentric circles centered on the point of light that produces them (Figure 28). The distance between consecutive circles represents the wavelength at a certain scale of drawing, for example, could represent a 1cm wavelength 0.000050cm.
Figure 28. The separation between spherical zones and wavelength changes from one place to another if the propagation speed changes.
If the wavelength change represented somewhere in the diagram, for example, because it changes the speed of propagation of waves, the distance between the concentric circles are also upset at points on the diagram for that place.
A mirage is the apparent reflection of bodies on the floor, as if on a water surface (Figure 29). It is observed in dies very sunny when the air is in contact with the earth’s surface heats up much more than that is in upper layers. The warm air below expands and becomes less dense than colder air above. This in turn decreases continuously the refractive index of air from the upper layers until it is in contact with the ground. The light emitted from an object into the soil changes its direction of propagation and ends also continually progressing along a curved path that eventually leads upward, as shown in Figure 29. An observer sees the object in the direction that has the light when it comes to your eye, that is, see it in the general direction of the floor as if there would be reflected light from the object.
Figure 29. A mirage is the apparent reflection of bodies, as if there is a reflecting pool on the floor. There are very sunny in days and due to the warming of the air layer in contact with the ground.
If we represent the light waves emitted by an object point by means of concentric circles, the distance between consecutive circles must change from one to another because the propagation speed of light also changes from an air layer to another. If the distance between circles increases from top to bottom, the direction perpendicular to the circles, which is the direction of propagation of light, curving upward as shown in Figure 30 (a), but if the distance between circles decreases, the direction of propagation curves downward as shown in Figure 30 (b). The mirages are formed because the direction of propagation curves upwards when the air becomes less dense. This shows, then, that if the light is a wave phenomenon spreading velocity must be greater in the least dense and reaches its maximum value when the density is zero, that is, in a vacuum.
Figure 30. (a) The direction of propagation of light bends upward if your speed increases in the lower layers. (b) The direction of propagation of light bends down if its velocity decreases in the lower layers.
This conclusion of the wave theory on the speed of light in media of different densities is strongly opposed to that obtained with Newton’s corpuscular hypothesis, which asserts that the propagation speed is higher in the denser medium. The dispute between the two theories could be settled by measuring the speed of light in media of different densities, for example in air and water, but the speed of light is so great that in times of Huygens and Newton was not possible to this measure rather than by astronomical methods and their value was known only in a vacuum, which is approximately equal to 300 000 km / s. The controversy lasted about 150 years until the experiments of diffraction and interference made by Thomas Young in 1815 gave much support to the ideas that measuring wave velocities of light in different media lost its importance to settle the controversy. However, in 1850 the French physicist Jean B. Foucault could measure the speed of light in water by finding a value 33% lower than in air. Newton’s corpuscular theory seemed to be definitely dead.
VII. ELECTROMAGNETIC WAVES
M any optical phenomena, such as diffraction and interference can be explained simply by the wave hypothesis of light without the need to clarify the ownership of the medium is perturbed and propagates in waves. Wave optics survived and developed for nearly 200 years, during which many optical problems were resolved and many optical instruments were developed without knowing the nature of the waves, only needed to know that they were waves. Nor was it necessary in many cases specify the nature of the medium in which light waves propagate; enough to assume that there was one capable of transmitting. This medium was called “ether.” However, to understand the nature of light was necessary to know the properties of the medium and determine which one is disturbed and wave propagates. Based on the values already known for the speed, The wavelength and frequency of light is determined that the ether of course should have special characteristics that made it unlike any other known means, such as air or water. For example, as was thought in light waves in the acoustic analogy, the ether would be a way analogous to air, but the frequency of light waves is billions of times greater than that of the acoustic waves, the ether was thousands of times more elastic than air, with properties similar to steel, to vibrate so quickly. It should also be transparent to let light and infinitely thin to allow unlimited movement of celestial bodies. All attempts for many years to prove the existence of ether were unsuccessful. No wonder now, after all, an extraordinary tool that has not ever been found.
While developed to achieve optical wave theory and are choked with the ideas ofether, other parts of science also rose. In particular, the science of electricity and magnetism had been developed independently from the elementary phenomena discovered centuries ago by the Greeks, skin rubbing amber objects and moving electrical charges produce iron objects without touching them, with pieces of a strange-magnetite-ore brought from the region of Magnesia in Central Asia until the experiments of Danish physicist Hans Christian Oersted in 1820 and the English physicist Michael Faraday in 1839, which showed a strong relationship between electricity and magnetism. These experiments proved that the electric charges generated by rubbing two bodies which attract or repel other charges generated in the same way can also attract or reject magnetized bodies, like a compass, but only if they are moving. That is, electric charges, in addition to electrical forces that attract or repel other electrical loads, If they are moving around also produce magnetic forces that move bodies such as compasses and magnets magnetized. This discovery, for one, gave rise to one of the most important inventions of modern civilization-the electric motor, which is essentially a device for circulating electric charges through a conductor of electricity and a magnetized body is put in motion by the magnetic forces generated by these moving loads. The same discovery, unexpectedly, also released the optical ether, their futile search.
The magnetic force produced by moving electric charges appearing around loads, where once there was no magnetic force them to start moving. It is a property of the medium, which changes when electric charges move. The magnitude of the magnetic force changes from zero, when the charges are at rest, to nonzero values, which reached when charge movesAnd depend on the speed of loads. In other words, the moving charges disturb the environment in a manner similar to the way in which the pressure and air density are disturbed by the vibration of a bell. It is conceivable, then, that the magnetic force produced by the motion of electric charges spread around the charges in a manner analogous to propagate changes in air pressure that constitute sound, ie waves. If the charges vibrate changing the direction of motion continuously, the magnetic force also produce changes in value and direction continuously, producing around areas of magnetic force with different values and directions. Thus, one can speak of waves of magnetic force produced by moving charges in the same way it speaks of acoustic pressure waves produced by vibrating objects such as bells or horns. These waves are called electromagnetic waves because along with the magnetic force also spreads the force produced by electrical charges (Figure 31).
Figure 31. The moving electric charges produce electric and magnetic forces that are propagated around the speed of light. The spread of these forces is called electromagnetic wave. At certain frequencies these waves are perceived as light.
The electric and magnetic forces produced in a certain place by moving charges do not appear instantly in that place to begin the movement of charge, but take some time. The speed of propagation of the magnetic force is equal to the distance between the charges and place, divided by the time it takes to receive magnetic force from the charges begin their movement. Of course, this is also the speed of propagation of electromagnetic waves. The Scottish physicist James Clerk Maxwell in 1865 found the way to calculate this velocity and obtained the value of 300 000 km / s. The same value of the speed of light! This could be a mere coincidence, and led Maxwell to testify that he had “strong reasons to conclude that light is an electromagnetic disturbance in waves …”. It seemed they had finally found the wave disturbance which constitutes light, and ideas about the ether were buried forever.
VIII. SOURCES OF LIGHT
L AS IDEAS Maxwell produced another result of enormous importance: they explained how light is produced. This is produced by moving electric charges. The simplest electromagnetic wave is produced by reciprocating the simpler system of charges. This system, called the electric dipole is formed by two electric charges equal but opposite signs, ie a positive charge and another the same but negative. Is the system más easy because our universe is electrically neutral and to produce an electrical charge of a sign there is always a load of opposite sign. Figure 32 shows, at one point, the electric forces that occur around an oscillating electric dipole. These forces vary at each point to the same frequency with the oscillating dipole moment. If a surface should join the ends of all the electrical forces, we would curl away from the dipole, the circular waves as in Figure 22 generated by disrupting the calm water surface.
Figure 32. The length of radio waves is hundreds of meters and are produced in systems of electric charges in motion, called antennae, which have dimensions comparable to about 100 to 200 m.
The radio waves and television are like these. Are generated by oscillating electric charges by the driver loads, usually vertical, called antenna. These waves differ from light waves in frequency only; The radio frequencies are among the millions and billions of hertz (megahertz, MHz, gigahertz, GHz), and light have frequencies of tens of billions of hertz (Hz tera, THz). Electromagnetic radio waves were produced artificially for the first time in 1887 by German physicist Heinrich Hertz, who also measured the speed of propagation and found that is equal to that of light, as Maxwell had predicted.
The waves produced by a luminous body, responsible for the phenomena of diffraction and interference, are produced by the movement of electrically charged particles that are built with atoms and molecules in the body. Because these movements occur in all directions and not just one as in a vertical antenna, the electromagnetic wave produced by each point of light is spherical and spread in all directions. The disturbance traveling consists of electric and magnetic forcesEthics produced by the moving charges and does not require a material medium to propagate, as it can do in the empty space. It is difficult to represent these waves by a drawing on paper because the electric and magnetic forces are perpendicular to each other and perpendicular, in turn, to the direction of wave propagation. Figure 33 shows how it would, at a certain instant, the magnetic forces of a spherical wave around loads that produce them. The force at each point varies continuously so that the areas they pass, for example, the maximum of forces away from the center to the propagation speed of light. Except that the forces are perpendicular to the direction of propagation, Huygens correctly postulated the existence of these waves to explain the phenomena of light refraction (Fig. 28), and Young to explain interference and diffraction (Figures 17 and 23) .
Figure 33. The magnetic forces on a spherical electromagnetic wave. The forces are moving at the speed of light occupying areas increasing.
All electromagnetic waves are generated by systems of electric charges in motion. In general, the wavelength produced is comparable to the dimensions of the loads, for example, radio waves have wavelengths of about 300 m and the transmission of radio antennas are also about 100 or 200 meters long (Figure 32). The length of electromagnetic waves, commonly called “microwaves”, is about 12 cm. The waves are produced in electronic instruments, called “magnetrons”, with these approximate dimensions. The call light electromagnetic waves have a length of about 0.0005 mm, indicating that microscopic systems are generated by electric charges of comparable size. These systems have an approximate diameter of 0.00001 mm and are called atoms or molecules. All matter consists of atoms or molecules of different species, all are composed of electric charges and thereforeAll are capable of producing electromagnetic waves. In addition to light waves, such systems could produce waves of greater length, called infrared radiation, and shorter waves called X rays and ultraviolet radiation, which can not be paid directly to the eye. In the interior of atoms and molecules are also present systems of electric charges much smaller, about one hundred thousand times smaller, so-called atomic nuclei that are capable of producing electromagnetic radiation of much shorter than ultraviolet radiation or the x-ray These electromagnetic waves are called gamma rays and can not be perceived by sight. When all the various types of electromagnetic waves is called the electromagnetic spectrum. Figure 34 shows its wavelength, frequency and size of systems that produce electrical charges.
Figure 34. The spectrum of electromagnetic waves and the dimensions of the charging systems that produce them.
IX. “Waves or particles?
The electromagnetic theory of light proposed by Maxwell answered questions and resolved major problems. The key was discovering that the disturbance traveling as light waves are formed by electric and magnetic forces, and that these disturbances are moving electrical charges. These discoveries led to radio, television and, in general, all modern telecommunications technology. It also seemed to have definitively established that light is composed of spherical waves (electromagnetic) that occur at each point of the luminous objects. However, in less than ten years found that this could not be entirely true, but rather the light, several new experiments, showed signs of being composed of granules or corpuscles and not continuous as are the interference wave.
The simplest experiment that shows the granular nature of light is simply to take pictures of an object to different degrees of exposure of the film, from very low to the right to get a good picture. Figure 35 shows the results of this experiment. The exhibition in the first picture was 10 000 times smaller than last, that is, that the amount of light reaching the photographic plate in the first picture was also 10 000 times lower than last. In the lower photo exposure time, (a) and (b) shows clearly that the film is printed by dots, as if the lamp is formed by granules or particles which arrive separately and leave behind her individual marks the film. This contradicts the idea of light as spherical waves because these are continuous, their effect on the film should also be continuous, and the image of the object should move gradually forming but complete and not all individual points as shown.
Figure 35. The particle nature of light is seen in photos of weakly illuminated objects. The image is formed point by point, and shows that the light reaches the film by separate units that produce them.
In the same pictures (a) and (b) one can notice that the points that form the image are essentially the same, just more points in the bright parts in the dark. This suggests that the assumptions granules or light corpuscles are also essentially the same as they produce the same effects on the photographic plate.
Each of these units is indivisible. This is also demonstrated easily with a variant of the same experiment. It bisects the light reaching the camera in the previous experiment using a semiespejo, ie a mirror that reflects half and transmits half of the light reaching it (Figure 36). Taking two pictures simultaneously, one with and one transmitted light with the reflected light, is that images in both pictures are composed of points identical to the photos in Figure 35. The only difference is that in this experiment the images are integrated into the double time. That is, the semiespejo simply halved the number of indivisible units of light coming into the first chamber and reflects the other half of the number of corpuscles in the second chamber.
Figure 36. Experiment to observe the division of light equally. The semiespejo reflects half the light reaching it, and transmits the other half.
The most impressive demonstration of the particle nature of light can get just now realizing how is the image of a star with a camera on a photographic plate. As in earlier experiments in Figure 37 shows that the image of every star in the film is integrated with the identical points of these experiments. Each of the indivisible units of light they produce these items thousands of years to travel 300 000 km / s, from one star to Earth, and the photographic plate produces a mark identical to that each unit makes light of an object placed a few feet from the camera. If the light produced by each of these brands had originated in the star as a spherical wave, to spread across the vast interstellar distances will beía distributed over a vast area in outer space coming to the camera so insignificant a part of it could not produce the photographic emulsion in an effect equal to that produced by each unit of the light coming from a nearby spring. Only by the rectilinear propagation of light corpuscles can understand this experimental result. This, largely vindicates the corpuscular hypothesis of light of Isaac Newton from the simple observation of the rectilinear propagation of light through the simple experiment of Figure 1. Indeed, one of the main arguments in support of Newton’s corpuscular hypothesis was that while “the sound of a cannon or a bell can be heard behind a hill you hide it,” because the sound is a wave disturbance that is spreads throughout the environment around the hill, “the fixed stars are no longer seen by the interposition of a planet”, unable to spread its light across it “as being composed of tiny bodies that are emitted by the luminous substances” .
Figure 37. The photographs of the stars are also formed pointwise, in the same manner as those of nearby objects. The corpuscles of light, or photons, producing such items thousands of years traveling through space to reach the film.
Light corpuscles also manifest their existence by their effects on certain systems that produce them. For example, light tubes that are usually called “neon gas,” but that are also made with other gases, producing light because the electrical charges, or electrons, the current that circulates through them collide with gas molecules filling the tube and shake the electrical charges within them, causing them to move briefly, like tiny bells, which vibrate at certain frequencies. Each excited molecule and produces light of characteristic colors for a very short time, a few billionths of a second, after which it returns to its original state. It also notes that the gas is heated during this process. This, in turn, indicates that its molecules acquire higher velocities than they had initially, because the temperature of a gas depends on the velocities of their molecules (Figure 38).
Figure 38. Production of light in a gas lamp. Loads of electrical current or electrons collide with gas molecules and electrical charges stir within them, causing them to move temporarily and produce light colors characteristic of the molecule.
Now suppose that the light is produced in spherical waves and that each atom, to produce light, is like a little cork that falls on water and produces a circular wave (Figure 39 (a)). Upon landing, the cork pushes water in all directions around and this forms the circular wave. Because in every direction pushes the cork remains in the same place where he fell, for, had pushed in one direction would have moved in the opposite direction as does a motor launch to push water in one direction with the propeller. But then would not have produced a circular wave, but a more or less “directed” in the direction of drive, similar to the wake that forms behind the motorboat to move (Figure 39 (b)). Reasoning now conversely, dropping many corks on water and we found that they had acquired speeds before falling conclude that somehow, each of them would have pushed the water in one direction, acquiring, as a result, a speed of recoil in the opposite direction.
Figure 39. (a) A cork falling into water produces a circular wave. The cork stays where it fell because it produces circular wave pushing water in all directions simultaneously. (b) If the cork pushed in only one direction, the wave generated would not move, but to be headed in this direction. In reaction to his push water, the cork back down in the opposite direction.
A similar problem to this Albert Einstein found in 1917 to account properly for the gas temperature reached when the molecules emit and absorb light is necessary to assume that each emission of light occurs at a specified address and not a spherical wave. Each transmitter molecule acquires a recoil velocity in the opposite direction of emission, producing precisely the molecular motion necessary to account for gas temperature. That is, Einstein showed that light waves are not spherical, but the light is emitted in precise directions as if composed of particles which Einstein himself called photons.
Einstein’s work also confirmed the first hypothesis of the granular composition of electromagnetic radiation. This assumption was made in 1900 by German physicist Max Planck to explain the wavelengths, ie colors of light emitted by incandescent bodies, such as a bar of hot iron. To account for exactly the amount of light emitted at the observed wavelengths, Planck found it necessary to assume that each frequency electromagnetic radiation is composed of indivisible units equal and that each contains an energy equal to the frequency of the wave multiplied by a number, now known as Planck’s constant and is represented universally by the letter h. That is, the energy E of each of the “packages”, or “quanta” of energy that make up the wave of frequency f is obtained from the so-called Planck equation “E = hf. This problem of physics, known as the “black body radiation” and the famous Planck equation gave rise to modern ideas of the granular composition, or corpuscular light.
X. To catch a photon
COMPOSITION granular light, demonstrated in multiple ways, some of which we described earlier, can not be doubted. It is also demonstrated by Einstein’s discovery that light is not emitted by spherical waves. However, the success of wave optics to explain the phenomena of refraction, diffraction and interference of light can not be ignored, nor may go unnoticed as the relation established Planck’s equation E = hf from “particles of light “, or photons, and a wave of frequency f. That is, the motion of corpuscles of light, or photons, must be associated with the electromagnetic wave determines its energy and, in many experiments, also determines its behavior.
How can the diffraction of light by a slot if we assume consist of indivisible photons? We saw that if we try to measure the diameter of the photons closing the slot to allow passage just find that they pass through the narrower slots, so that you can not assign a defined scale. However, some property of the photon must be related to length since it distinguishes a narrow groove in which is diffracted from a wide slot through which passes undeflected. Wave optics, meanwhile, provides that a slit diffraction occurs only if its width is comparable with the wavelength of light, ie if the width is at most, a few times the wavelength. This implies that some property of the photon must be related to the length of a wave. Planck’s equation E = hf, shows this relationship. If we write the frequency f and the speed of light c divided by the wavelength L, the Planck equation is written in the form E = hc / L that relates a property of the photon energy, with the length of a wave . This wave must be the electromagnetic wave produced by moving electric charges, together with the photon.
We can now attempt to make us a more complete representation of what happens in the production of a photon. According to Einstein’s result, the photon is produced in a very short time as a billionth of a second, the momentary excitement of electric charges of a molecule, it is also delivered at the speed of light, but not as a spherical wave, but a well-defined direction. Simultaneously charges produce an electromagnetic wave is propagated, also the speed of light, along with the photon. The wave may be spherical or not, this depends only on the motion of the charges, but its extent is limited. The wave occupies only the distance that light travels during movement of loads. For example, if the emission time out of a billionth of a second extension of the wave would be a billionth of the 300 000 km light travels every second, that is, would be 30 cm. A wave of limited extent is called”wave train” and is represented as in Figure 40. As the wavelength of light is so small, on the train can fit many complete waves. If the wavelength of light in the above example out of 0.00005 cm, on the train would fit 30 / 0.00005 = 600 000 complete waves. The frequency of the wave is the number of oscillations in one second, that is, f = 600 000 / 0.000000001 = 600 trillion hertz. The emitted photon energy depends on the frequency of this wave.
Figure 40. A train of waves is a wave of limited extent.
The electromagnetic wave train is generated with the photon, but not the photon. This can be found anywhere in the region occupied by the wave train, but is not distributed on the entire region, if found somewhere, it is full. For example, if the train was spherical, the photon could find, track, anywhere in a spherical shell of 30 cm thick and with a radius that increases the speed of light. In this shell is the train of waves generated by the movement of loads (Figure 41). In other words, the electromagnetic wave train shows wherecan be found, but where is the photon. For example, if the spherical wave train reaches a thin slit, behind her and does not propagate as a spherical wave, but due to interference in the transmitted wave, form zones of destructive interference, where the electromagnetic wave is canceled completely, and areas of constructive interference where the interference increases the intensity of the wave. These are the areas that are seen in the diffraction of light through a slit (Figures 17 and 18). If the slot is illuminated with light so weak that the photons pass through it one by one, each of them could be found in places of constructive interference, but can not be found in areas of destructive interference. This can be proven to receive the photons in a photographic plate. The points marked on the film and the transmitted photons fall, one by one, at points of constructive interference of the wave and no point falls in places where the interference cancel the intensity of the transmitted wave. Moreover, the points accumulate faster in the central band of the diffraction pattern on the sides, indicating that photons prefer to reach the places where the electromagnetic wave is more intense.
Figure 41. A spherical electromagnetic wave train produced by a photon generated by the movement of charge in an atom. The photon can be found, complete train anywhere with equal probability.
The experiment of Young’s double slit the photon finds this relationship with the electromagnetic wave. The illumination pattern of the double slot differs fundamentally simple slot in interference bright images that appear at the center (Figure 21). We need now to explain how these images are interfering with the photons that make up the light reaching the double slit. Since photons are indivisible, each photon passes only one of them. If the slot through which a photon does not pass had no effect, all passing through a slot would produce a diffraction pattern produced by independent who pass by each other and there would be no interference between them. Be observed only two overlapping diffraction patterns of a slot and central bright images not observe constructive interference. The two slits each photon influence because both transmit the electromagnetic wave associated with each. Behind the slits are then produced areas of constructive interference where a photon passing through any slotcan be found, and areas of destructive interference where any transmitted photon can not be found. Each transmitted photon is more likely to reach the central regions where the intensity of the transmitted wave is large due to the constructive interference, and less likely to reach the middle and side, where the intensity of the transmitted wave is small due to destructive interference.
Still one would think that the central images observed with slot resulting from direct interactions between photons that are in transit through the slots. That is, photons, somehow, alter each other’s movements cluster in some regions and away from others. This possibility was eliminated in 1909 by the British physicist GI Taylor, who conducted the experiment of Young’s double slit light sources extremely weak, so that the photons passing one by one by the double slit and do not existthe possibility that it influenced each other’s movements. The transmitted photons were received in the film of a camera printing each point on the site of arrival. Accumulating points for a very long time, sometimes for several weeks, was found to always form the same images of interference seen with more intense light sources.
This showed that the images may not be of mutual influences between photons and supports the hypothesis that each transmitted photon can reach any place where the intensity of the wave does not vanish. Photons, of course, more likely to reach the areas where the intensity of the transmitted electromagnetic wave is large and not reach the areas where this intensity is canceled by destructive interference.
To catch a photon should therefore be sought in the areas of greatest intensity of electromagnetic wave. For example, to catch it after passing through a double slotWe can make a Young diagram of waves transmitted through the slits, as in Figure 23, and identify areas where they interfere constructively, these are the darkest regions that are looking at the diagram at an oblique angle from the right side to the left of Figure 42. A photon transmitted by any of the slots can be caught with high probability, in any of these areas. It is not possible, however, know in advance what will be found, it can reach any of them with equal probability. Nor is it possible to ensure, once trapped photon, which of the two slots was transmitted, could be either with equal probability.
Figure 42. Young diagram to find the areas of constructive interference of waves transmitted by two slots. These are the dark regions that are noted obliquely viewing the diagram from the right side to left. A photon of light transmitted by the grooves can, in all probability, any of those areas.Figure 42. (b)
This example shows that it is generally not easy to know where to place areas of constructive interference of waves encounter obstacles because the waves transmitted, or reflected, are combined in space by creating areas of interference of complicated forms. The example of the double slit is the simplest and requires drawing a scale diagram of the position of the slots, and many equidistant concentric semicircles representing waves transmitted through the slots. The areas of constructive interference of waves transmitted by two or more slots can easily find games semicircles drawing equidistant from each slot, clear plastic sheet as shown in Figure 43. For example, the zones created by a double slit are observed by superimposing two such sheets in a white paper so that the foundations of the semicircles are in a straight line with the centers slightly apart. The areas of constructive interference of waves transmitted están represented, as before, the dark regions are formed at intersections of the semicircles. Changing the separation between the centers of the two sets of semicircles can see how they change the regions of constructive interference to change the distance between the grooves. This method can also be used for more than two slots, more overlapping plates, or slots that are not on the same plane, laying the foundation of the semicircles on straight lines form an angle equal to the planes of the slots.
Figure 43. Young diagram for two slots, built with transparent plastic sheets superimposed on white paper. Each sheet represents the spherical wave propagated from a slot with a series of semicircles equidistant. Changing the separation of the centers of the plates shows how changing areas of constructive interference of waves transmitted.XI. The photon
FTER the preceding pages might answer “light is made of photons” with a good chance to understand the same with the word “photon”. Then summarize the main physical properties.
The photon is an indivisible particle that moves, always at the speed of light. This is the maximum possible speed of propagation in the Universe. No body can achieve material because the resistance of matter to be accelerated, its inertia increases with speed, and becomes infinite at the speed of light. To achieve this speed would be necessary to that body a force of infinite magnitude, which does not exist in nature. The photon moves at the speed of light because it is not a material particle, its mass is zero. This has the additional consequence that its speed can not be diminished, that is, photons can not be stopped, there are only moving at the speed of light. As ademore we can not move at that speed is impossible to stop or reach a photon for consideration. No sense even to imagine a physical aspect, if it is round and seamless as baseball, or plain, white with a black spot like a billiard ball. The points that appear in the low exposure photos in Figure 35 are not photons, but the traces they have left to transform into metallic silver salt crystals that absorb this metal. The possible existence of massless particles moving at the speed of light was advanced by Einstein’s theory of relativity. Therefore it is called “relativistic particles. There are other relativistic particles with different properties to the photon. Neutrinos, for example, are invisible to the human eye.
The photons are produced by moving electric charges. The electric charges produce electric and magnetic forces simultaneously propagating in the space at the speed of light as waves Electromagnethical. Photons move in specific directions, but were found only in places where waves occurring electric and magnetic forces generated by the loads. You can find a photon, the entire whole, wherever those forces there, most likely in areas where these forces are greater. As the electromagnetic forces are spread in waves, the photon will most likely be found in places of constructive interference of these waves and less or no chance in those destructive interference. This, in some phenomena such as diffraction, makes his motion to be confused with that of a wave, but the photon always appears as an indivisible unit and never in fractions, or spread on the region occupied by the electromagnetic wave.
The photons are manifested as particles, and concentrate their energy, their movements and their impact on regions defined and separated. In a photograph produced marks located as if the energía of each photon, which transforms the photographic emulsion crystals were concentrated in a small package. In fact, the first step of this transformation is a collision between a photon and a particle of electric charge of the crystal, an electron, which emerges from this a result of the impact as if it were the clash of two marbles. This phenomenon, called “photoelectric effect” finds a great job in producing electric current by light in the so-called photoelectric cells (Figure 44). Noting the trajectories of electrons colliding with photons is that these crashes occur exactly as if the electron and photon were two billiard balls, ie, the angles of the trajectories and energies of the two particles before and after the collision are identical that they would be microscopic two billiard balls with the same energies (Fig. 45). This phenomenon, called Compton effect, is the best sample photons as particles in the sense of a marble or billiard ball.
Figure 44. The photons of a light beam of an electric charge torn metal electrons colliding with them, communicating motion and energy as if it were collisions between marbles. This phenomenon, called photoelectric effect, is the first stage of the photographic process, the photon to an electron starts a salt crystal silver of the photographic emulsion. Figure 45. The trajectories that follow an electron and a photon colliding are identical to those that would follow two microscopic billiard balls have their same energies. This phenomenon, called Compton effect, light clearly shows its particle aspect.
Strange the mechanical properties of photons. In some cases closely resemble baseballs or billiard balls, but others are very different. If we try to confine the path of a photon through a thin slit we find that the possible paths that can follow afterés passed through it will multiply and can be found in many places across the slot. This is because photons are always created with electromagnetic waves and can be found anywhere where they arrive. If the waves are diffracted in a slot, each photon can reach any part of the diffraction pattern of the waves. There are no waves photons because the very nature of light is dual; has both wave and particle simultaneously and, although it depends on what you do with the light which of the two types of ownership is apparent, will always display the other side somehow. For example, to describe the collisions of photons the wave aspect is shown and it is necessary to express the photon energy, since it inevitably involves the frequency of a wave. The wave and particle aspects are in fact complementary. The modern theory of light gives precision to this complementarity to make the two inseparable aspects of the mathematical description of the radiation by electrical charges. The descriptions and predictions of light phenomena obtained with this theory fit surprisingly well to the experimental facts, and support the basic ideas above. Perhaps one day is a phenomenon that destroys or modifies these ideas on light and photons, but so far have withstood all tests.
It is interesting to note also that the first basic ideas about light were never really abandoned. The rectilinear propagation was always support the particle theory, and it was the diffraction wave theory. Electromagnetic waves are not light, but correctly described its spread in space. In a sense, the distinction between electromagnetic waves and photons is the contemporary analogue of the distinction, in ancient times, made Alhazen between sight and light.
GLOSSARY
Acoustic. Part of physics that studies the phenomena associated with sound.
atom. Each electrically neutral particle that is composed of matter. It consists of a positively charged nucleus surrounded by an equal amount of negative charges.
electric charge. See electricity.
electrical current. Electric charges parallel movements, for example by a wire, forming an electrical current. We produce electric current daily connecting filament of a light bulb to the outlet is switched on, but only 150 years ago no one knew how to produce and only known in the spokes of the storms.
electricity. 2 500 years ago the Greek philosopher Thales of Miletus discovered that a bar of amber rubbed with fur attracts small objects such as bits of paper. Called electricity to property acquired by the bar because amber in Greek iselektron. The phenomenon is also observed in many other materials like plastic or glass and modern electric charge is called to the property acquired with the rub. The electric current we use every day consists of moving electric charges, which occur in more efficient ways to rubbing bodies.
electron. The electrical load consists of indivisible units of two different types, which are called positive electrical charges and negative electric charges. Electron is called the indivisible unit that contains the minimum negative electrical charge.
energy. In everyday language it is said that someone has power if it can do much work, such as climbing stairs. In scientific energy and work are related in the same way, but work means just moving a force, for example lifting a weight.
Energy can exist in different forms. There is in chemical form in gasoline and electrically in a battery or a battery supply. Whatever its form, energy can do work well, by burning gasoline engine to move the energy is used to do work.
hypothesis. Initial assumption, possible or impossible, to draw the logical consequences. For example, before the Copernican astronomy main hypothesis was that the earth occupied the center of the universe.
law. In the language of science a law is a statement that describes how a natural phenomenon occurs. A law is the result of observation, experimentation and deduction on them, eg the law of falling bodies says that without air resistance and from equal heights all bodies fall with the same speed.
magnetism. It is named the property possessed by magnets to attract bodies made of materials such as iron or nickel, and exert forces on moving electric charges or the lead wires as electricity.
The origin of this word is the name ancient Greeks gave to the magnets, or magnets, known to them. These were pieces of a mineral now called magnetite, which were in a region of Asia Minor called Magnesia.
molecule. Minimum amount of a substance that retains its chemical properties. In general, a molecule consists of a few atoms, and are called diatomic, triatomic and polyatomic under which consist of two, three or many atoms, respectively.
optics. Originally, the part of physics that studies the phenomena related to light. Its origin is Greek for visible, “Opto” which still meets the meanings of light and sight. Recently, the optical refers more specifically to science and manufacturing technique of auxiliary instruments for the eye, such as lenses, microscopes and telescopes, or the part of physics that deals with this subject.
perspective. Art of representing objects on a surface in the form and arrangement as they appear in sight. The laws of perspective were discovered in the Renaissance by the Italian architect Filippo Brunelleschi (1377-1446). They are based on the law of variation of the apparent size of bodies with distance.
pressure. Force exerted by a liquid or a gas on each cm ², m² each of the surface of a solid which is in contact.
The pressure exerted by the surrounding air is called atmospheric pressure.