Question

(i) Define the term refractive index of a medium in terms of velocity of light.

(ii) A ray of light moves from rarer medium to a denser medium as shown in diagram. Write down the number of the ray which represents the partially reflected ray.

Answer 1

(i) The absolute refractive index of a medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium i.e., $n=\frac{c}{v}$

Refraction is the measure of the bending of a ray of light when passing from one medium into another. If $i$ is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal) and $r$ is the angle of refraction (angle between the ray in the medium and the normal), the refractive index $n$ is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction;

i.e., $n=\frac{\sin i}{\sin r}$

Refractive index is also equal to the velocity of light $c$ of a given wavelength in empty space divided by its velocity $v$ in a substance, or:

$n=\frac{c}{v}$

(ii) The ray which represents the partially reflected ray is Ray $2$. Here the ray (Ray $1$) is the incident ray. Ray $3$ is the refracted ray and ray $2$ is the partially reflected ray.