The reason is because the definition of refractive index n of a medium (eg. glass) is the ratio of the speed of light in a vacuum c to the speed of light in the medium v.
Nothing can exceed the speed of light in a vacuum. Therefore the maximum value of n is 1 which is the refractive index of a vacuum or air (for which the refractive index is about the same as for a vacuum). When a light ray enters a medium from air or a vacuum the speed must decrease because v < c and so n <1 because n=c/v.
The post Why is the index of refraction always greater than or equal to 1? appeared first on homeworkcrew.
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