A student is studying a new material measures that the index of refraction of a certain material is 0.9. Is this a reasonable measurement? Why?

Question

Honors Physics 1 (Refraction)

Answer 1

No, the index of refraction for a medium cannot be less than 1, for the following reason.

the index of refraction is defined as a ratio:

the speed of light in a vacuum DIVIDED BY

the speed of light in the medium.

Since light cannot travel faster than it travels in a vacuum, this ratio cannot be less than 1.