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more matter particles on one side (water) than on the other (air), and light corpuscles experience an attractive force towards the water. That is, the light particle experiences a brief attractive force towards the medium that is denser, or has more matter particles (water), and so the vertical speed of light is greater in such substances. This brief vertical force speeds the light particle up and deflects its velocity towards the surface normal.19 In other words, 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 a medium of greater optical density, like water or glass, suggesting that the speed of light in water is greater than in air.
The diagrams below show that the ratio of sines is equal to both, the index of refraction and to the ratio of the speed of the light particles in water to that in air. In a way, all Newton needed to do was to posit that the speed of light is different in different transparent matter. This is a simple explanation of Snell’s Law.20
To explain reflection, Newton argued that steel ball bearings thrown at a smooth steel plate rebound in the same way as light reflects. When perfectly elastic particles bounce off a rebounding surface, they give a good model for the reflection of light. In this way, the incident ray and its angle equal the reflected ray and its angle of reflection.21
19 http://galileo.phys.virginia.edu/classes/609.ral5q.fall04/LecturePDF/L20-LIGHTII.pdf on Thursday, October 25, 2012. 20 http://galileo.phys.virginia.edu/classes/609.ral5q.fall04/LecturePDF/L20-LIGHTII.pdf on Thursday, October 25, 2012. 21 http://boomeria.org/physicstextbook/ch12.html and http://library.thinkquest.org/3227/webpage/particle.html
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