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The Direction of Bending:
Refraction is the bending of the path of light as it passes across the boundary between two media. The direction of refraction is dependent upon the relative speed at which light moves within the two media. The rules are:
- When light passes from a medium in which it travels faster into a medium in which it travels slower, the light ray will refract towards the normal line.
- When light passes from a medium in which it travels slower into a medium in which it travels faster, the light ray will refract away from the normal line.
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Optical Density, Speed of Light and Index of Refraction:
The speed at which light travels through a medium is dependent upon the optical density of that medium. Light travels fastest in media that are least optically dense. The index of refraction value provides a relative measure of the optical density of a material. The higher the index of refraction value, the more optically dense a material is and the slower that light will travel in that material.
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Consider the case of light crossing the boundary from a medium in which it travels relatively fast to a medium in which it travels relatively slow. Which way will the light refract at such a boundary? See the first Know the Law section. As the light bends away from the normal, the angle between the light and the normal gets bigger. Thus, it is biggest in the medium where it travels slowest. That's good logical thinking. Now use the second Know the Law section to associate the medium of slowest speed with the optical density and the index of refraction value. Taking your time and using a little more logical thinking will put another point up on your scoreboard.
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