When light enters a new medium at an angle, the change in speed causes the light to bend, or refract. For example, when light passes from air into glass or water, it slows down. When light passes from glass or water into air, it speeds up. The amount by which the light refracts as it passes from one medium to another depends upon the difference between the speeds of light in the two media.

Figure 6 shows how the path of a light ray changes as it passes from one medium into another. The incident ray of light, traveling through air, first strikes the boundary between the air and the water. As the light ray enters the water, it is refracted. You can see in Figure 6 that the light ray is now traveling in a new direction. As the ray enters the glass, it is refracted even more. Finally, when the ray reenters air, its path is bent again, but back to its original direction. Note that regardless of the refraction that occurs in the water and glass layers, the ray again travels in its original direction when it reenters the air.

How much the speed of a light ray slows as it enters a new material depends on the material's index of refraction. The index of refraction for a material is the ratio of the speed of light in a vacuum to the speed of the light in the material. A material with a low index of refraction (near 1) causes light to slow and refract very little. Air, with an index of refraction of 1.0003, is such a material. Diamond, however, with an index of refraction of 2.42, causes light to slow and refract significantly.

Figure 6 A light ray bends (refracts) as it passes through media with different indices of refraction. Inferring Based on the path of the light ray, which medium has the greatest index of refraction?

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