Alight beam has speed c in vacuum and speed v in a certain plastic. the index of refraction n of this plastic isn=cvn=(v/c)^2n=v/cn=c/vn=(c/v)^2

Explanation:

The index of refraction of a material is defined as:

where

c is the speed of light in vacuum

v is the speed of light in the material

The speed of light in the material, v, is always smaller than the speed of light in vacuum, c: therefore, this ratio is always greater than 1. The index of refraction basically tells how much is the light slowed down in a material: the greater the value of the index of refraction, the slower light travels in that medium.

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