Fermat’s principle

1662 Fermat’s principle

The book of science

Tom Sharp

Pierre de Fermat optics

Fermat’s principle

A modern explanation


Light has a constant speed, c, only in a vacuum. In other transparent materials, light travels more slowly. The ratio of the speed of light in a vacuum to the speed of light in a material is called the refractive index. For example, in glass light travels about two-thirds of c, so its refractive index is about 1.5.

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