Question

Why is the index of refraction always greater than or equal to 1?

Short answer

The refractive index is defined as the ratio of the speed of light in vacuum c to the speed of light in medium v. Because the speed of light in any medium is always less than the speed of light in a vacuum, the refractive index is greater than${}^{\mathbf{1}}$.

role="math" localid="1654058921693" $\mathrm{n}\ge \mathbf{1}.$

Whereas it equals${}^{\mathbf{1}}$light in free space.

Step-by-step solution

Definition of refraction

Refraction is the change in the direction of a wave as it travels from one medium to another or as the medium gradually changes. Although light refraction is the most commonly observed phenomenon, other waves, such as sound and water waves, also experience refraction.

Explanation

The refractive index ${}^{\mathrm{n}}$ is defined as the ratio of the speed of light in the vacuum ${}^{\mathrm{c}}$ to the speed of light in the medium ${}^{\mathrm{v}}$.

role="math" localid="1654059167451" $n=\frac{c}{v}$

And, because nothing is faster than the speed of light in vacuum ${}^c$, the maximum value for the refractive index is actually when it moves through free space, which is one. Otherwise, the refractive index is greater than one.

Thus,${}^{n\ge 1}$.