What is the refractive index of a medium?

A. the ratio of the velocity of light in the medium over the velocity of light in a vacuum

B. the ratio of the velocity of light in a vacuum over the velocity of light in the medium

C. the ratio of the angle of incidence over the angle of refraction

D. the ratio of the angle of refraction over the angle of incidence

Answer: Option B: the ratio of the velocity of light in a vacuum over the velocity of light in the medium, n = c/v

Explanation:

When a wave of light enters in some materials, the velocity changes depending on the material, and this is why some times when light enters in something, for example, a glass of water, the "path" of the light changes (and you can see some cool visual effects)

then, we define the refractive index of a medium as:

n = c/v

where n is the refractive index, c is the velocity of the light in the vacuum and v is the velocity of the light in the material, here you can see that n is always greater or equal than 1 (in the case n = 1, we also have v = c)

Then, the correct option is:

option B: the ratio of the velocity of light in a vacuum over the velocity of light in the medium

The refractive index is the speed of light in a vacuum over the speed of light in the medium.

The answer is B. the ratio of the velocity of light in a vacuum over the velocity of light in the medium