Newton’s Corpuscular Theory of Light: Reflection and Refraction
Newton’s Corpuscular Theory of Light
According to this theory, developed in 1671 by the English physicist Isaac Newton, light was the projection or release of small material corpuscles (Newton called them “hits”) from the light source. These corpuscles are propagated at high speed in a straight line (because of their inertia) through all semi-transparent and homogeneous media.
Reflection Explained by the Corpuscular Model
This particle model successfully explains not only the rectilinear propagation of light but also the reflection that occurs as a result of elastic collisions of light corpuscles with the surface of illuminated bodies. If there is no friction, the tangential component of particle velocity does not change. However, the surface normal component is reversed because of the huge mass difference between light particles and the illuminated body. As a result, the angles of incidence and reflection are equal.
Other Luminous Phenomena
Corpuscular theory also allows an understanding of other luminous phenomena discovered subsequently, such as the photoelectric effect and the Compton effect.
Refraction and the Corpuscular Theory
With respect to refraction, Newton, to explain the sudden change in speed experienced by light passing from one medium to a different one, supports the existence of forces acting at very small distances, in the vicinity of the boundary of the two media, between the corpuscles of light and the material atoms. These forces do not affect the tangential component of velocity, but the normal component, which results in a change in the direction of the beam when crossing the boundary. Thus, when light passes from air to water, it changes direction, approaching the normal, fulfilling, according to the figure, the following relationships:
sin i / sin r = v1/v2
as v = v1x / v2x, it is finally
n1 sin i = n2 sin r – where n2 is the refractive index of medium 2 and n1 the refractive index of medium 1.
Speed of Light and Foucault’s Experiment
In this way, as i > r, it must be fulfilled, if we accept the corpuscular theory, that the speed of light in water is greater than in air. At the time of Newton, there were no means to measure a speed as high as that of light, which would prove or disprove the above expression. It was not until 1862 when the French physicist Léon Foucault proved experimentally that the speed of light, contrary to the assumption of Newton, was lower in denser media, acquiring its maximum value when it propagates through a vacuum.
Limitations of the Corpuscular Model
Besides refraction, the particle model “fails” in interpreting other luminous phenomena such as diffraction and interference. However, the prestige of Newton made the corpuscular theory accepted by most scientists of his time, and for more than a century, it maintained its supremacy in front of the wave model proposed by Huygens.
Total Internal Reflection
When n1 > n2, sin i < sin r and therefore i < r. It happens then, that there is a certain angle of incidence, called the critical angle, which accounts for refraction, r = 90º. If you send a ray with an incident angle greater than the limit, refraction does not occur, but that beam is reflected at the surface separating the two media, following the law of reflection. In other words, the surface separation, in this case, behaves like a mirror. This behavior is called total reflection.