Corpuscular Theory
Newton believed that light was made up of billions of tiny particles he called "corpuscles", which came out of whatever was giving off light. Let's examine the arguments used by people who believed in the particle theory.
A ball thrown into space follows a curved path due to gravity. But, the greater the speed at which the ball is thrown, the less and less its path curves. We can easily imagine these little corpuscles traveling at such high speeds that they travel in straight lines.
Newton had no problem in explaning rectilinear propagation with his theory. In fact, this property of light provided the supporters of his theory with one of their strongest arguments against the wave. How could waves travels in straight lines? A sound could be easily heard around a corner, but light can't be seen from behind a wall. The former observation is unquestionably a wave phenomenon. How can the latter be, too? The simple and direct explanation of rectilinear propagation provided by the particle of light model, along with the prestige of Newton, were largely responsible for the preference of the particle theory in the 17th and 18th centuries.
Newton was able to demonstrate the nature of refraction with his particle model. We can duplicate this experiment. First we need two level surfaces, one higher than another, with their neighboring edges connected by a slope. If you roll a ball along the higher surface, down the slope and across the lower surface, the ball will have sped up. This added speed is, of course, due to gravity.
Suppose the ball is rolled on the higher surface toward the slope at a given angle. At the slope, the accelerating force pulls on the ball causing it to roll across the lower surface at a lower angle. Now if we think of the upper surface as air, the lower surface as a more dense medium like water, and the slope as the interface of the two media, the rolling ball behaves as particles of light being refracted as they pass from air to water.
Newton believed that water attracted the approaching particles of light in the same way that gravity attracts the rolling ball. The rolling-ball experiments imply that light particles accelerate as they go from air to medium of greater optical density, like water or glass. But, if that was the case then the speed of light in water would have to be greater than in air. Newton knew that if it should be proven that the speed of light is slower in water than air, then his whole theory would be shot to pieces. In fact, 123 years after his death, a French physicist, Jean Focault (that's foo-koh), proved that light travels slower in water. This condition was proved by the wave theory.
