Exploring the Wonders of Light: Reflection, Refraction, and Color Perception
Reflection
When light travels through a medium and encounters a surface that separates it from other mediums, some of it is reflected and continues propagating through the same medium, changing direction and obeying the laws of reflection:
- The incident beam, normal, and reflected ray are in the same plane.
- The angle of incidence and the angle of reflection are equal.
A Kaleidoscope
A kaleidoscope (from the Greek kalós eidos, beautiful image, and scopéo, observe) is a tube containing three mirrors that form a triangular prism with the reflecting sides facing inward. At one end of the tube are two translucent sheets between which are placed several objects of different shapes and colors.
Our Image
What happens when we face two mirrors that are 90 degrees apart?
With two mirrors, a double image reflection is produced, and the side where the image is reversed is avoided.
Infinite Images
What happens when two parallel mirrors face each other?
In one mirror, we see our image in front, and in the other, our image appears behind, which in turn is reflected in the former and so on, producing an endless succession of alternating images (front, rear, etc.).
How Many Reflections Occur?
With two mirrors joined by a piano hinge, we can observe how the angle formed influences the number of reflections. By placing the mirrors on paper and drawing a line, we can question what polygon will be formed for each angle.
- 120° angle – triangle
- 90° angle – square
- 72° angle – pentagon
- 60° angle – hexagon
“Black Body”
The outside is black, what about the interior? Look inside through the hole and then open the box. Can you explain what happens?
From the inside, no light is emitted. The multiple reflections make the light returned to the outside very small. Therefore, we are unable to see the white.
Refraction
When light travels through a medium and encounters a surface that separates it from another medium, some of it is transmitted through the other medium, changing direction because light propagates at different speeds in each medium.
When entering a denser medium, light slows down and bends towards the normal. When it passes to a less dense medium, its speed increases, and it bends away from the normal. Only in the latter case can total internal reflection occur, which happens when the angle of refraction is 90°.
The refractive index of a medium is a property of the medium itself and is directly related to the speed of propagation of light in that medium. The lower the speed of propagation in the medium, the higher the refractive index.
Refraction follows Snell’s law, which justifies that if the index of the medium that light passes into decreases in value, the beam moves away from the normal; otherwise, it approaches the normal.
To See or Not See
Put a coin in the center of a bowl and move back, away from the bowl, until you can no longer see the coin. Then ask a friend to pour water very slowly into the container. What has happened?
(The experiment can be repeated in another bowl with another coin, but adding a solution of CaCl2 instead of water. CaCl2 has a higher refractive index, and the coin will become visible by adding less liquid.)
When the light from the coin exits the water, there is a change in the direction of light (a phenomenon called refraction of light) that causes us to see the object above its actual position.
Looking for Invisibility
When two transparent media have the same index of refraction, the light does not appear to be refracted as it passes from one to the other.
Dry Sand, Wet Sand
Wet sand with different substances appears more or less dark. The substances used are: distilled water, calcium dichloride solution, carbon disulfide, and toluene.
Fine sand is brighter and clearer than coarse sand because, in the latter, photons (particles of which light is composed) are more likely to be absorbed. The scattering of photons depends not only on grain size but also on the substance that surrounds it and its refractive index. The higher the index, the lower the scattering angle, and therefore the light stays longer within the substance. This explanation can also be made by considering light as a wave.
Optical Instruments and Effects
The Camera Obscura
The camera obscura, the forerunner of the camera, of Arab origin and rediscovered by Leonardo da Vinci, has a hole that acts as a converging lens and projects an inverted image of the outside world onto the opposite wall, both vertically and horizontally.
A Glass Jar as a Lens
A glass jar or cylindrical shape, if filled with water, can be used as a converging lens. The refractive index of the liquid in each container will cause the lens to have different properties. When placing an object in front of a lens, the characteristics of the images formed depend on the distance at which the object is placed.
Why are the words in blue reversed and the red is not?
All the words are reversed, but those written in red have horizontal symmetry (C, H, and I).
What color is the oil slick?
We put a piece of paper on a table and drop a few drops of oil in the center to form a small spot. When exposed to light, it will be a dark spot on a white background. But if you put the paper against the light, you will see a white spot on a dark background.
The oil-impregnated paper is more transparent. If the paper is exposed to light, the stain can be seen as dark because it allows light to pass through and does not reflect it. But if the paper is held against the light, the spot is white and bright because the light passes through the transparent oil but not the paper.
Magic Cellophane
Simply place a piece of cellophane on frosted glass to suddenly make the light pass through more clearly.
Frosted glass has a rough surface, which causes light to scatter in all directions, and no image is formed on the other side. By putting the cellophane tape on one side, the adhesive fills in the rough surface, effectively “polishing” it.
Light and Color
Spectroscope
A spectroscope is a device designed to separate the different components of an optical spectrum. To produce the breakdown of a compound light of different colors (frequencies), Newton used a prism. In our case, we can use a diffraction grating, such as a CD, in which for every inch there are over 500 pits that cause scattering in all directions of all colors of the incident light beam.
Color Blindness Test
If you have not taken the test… This is the time. It is a genetic defect that is manifested by the inability of some photoreceptor cells in the eye to distinguish certain colors.
Named after the British chemist John Dalton (1766-1844), who suffered from this flaw, it makes it impossible to visualize certain colors. As with hemophilia, color blindness is a sex-linked (X chromosome) condition.
The retina is responsible for transmitting any information of light reaching it through the optic nerve to the brain. There are cells called cones, of which there are 3 types, to distinguish the different wavelengths associated with colors. The anomaly in these cones is the cause of color blindness. The most common type of color blindness is red-green.
Fluorescence and Phosphorescence
When the electrons of some atoms are excited to higher energy levels, they do not always return directly to their ground state. If the de-excitation process is almost instantaneous (less than 10-7 s), it is called fluorescence; if it lasts longer, it is called phosphorescence.
These processes are better perceived by illuminating the atoms with ultraviolet light (in this case, purple), which has the most energy. Then, the atoms, in the process of de-excitation, emit light of a lower frequency, therefore visible.
It is Not Always the Color that it Seems
(In the dark) The colors we know are identified with white light. When the light changes, the color of an object illuminated by that light also changes.
When an object illuminated by white light has a certain color, it is because it absorbs all colors except the color that is reflected if the material is opaque or transmitted if it is transparent (like the cellophane we use). This process is called selective absorption.
But shades are produced by additive mixture, for which we must bear in mind that white = red + blue + green. The yellow cellophane absorbs blue and passes red and green, which changes the color of balloons.
Dominant Eye and Stereograms
All of us have a dominant eye or director, with greater visual acuity, which is responsible for mastering the vision of depth, while the other is responsible for peripheral vision and spatial perception, mainly by extending the two-dimensional image to our brain.
The stereogram is an optical illusion based on the ability of the eye to capture images from different viewpoints. These different perspectives are captured in such a way by the brain that it appears to be a three-dimensional image.
The Moiré Effect
The Moiré effect is an optical effect produced by superimposing two grids of lines with a certain angle or when they are of different sizes. The origin of the word comes from a well-known silk weaving.
Polarization
Polarization is a phenomenon unique to transverse waves, and light is an electromagnetic wave in which electric and magnetic fields are perpendicular to the direction of propagation. Through suitable filters, we can make the light spread in a particular plane (plane polarization), and if we hinder the transmission in that plane, we get darkness.