Question

Which is the position of minimum deviation in prism?

Answer 1

Refraction through a prism:

1. Refraction is a phenomenon in which a change in direction or bending of a light wave occurs when light travels from one medium to another. This is caused by the change in the wave’s speed due to the change in the optical density of the medium.
2. Prism is a closed solid having two triangular bases and three rectangular surfaces. This has two refracting surfaces which means the surface where refraction of light takes place.
3. Let A, B, and C be the glass of the prism with AB and AC as refracting surfaces as shown in the figure. when the light ray OP refracts through AB, it bends and follows path PQ, which is called a refracted ray. As the ray PQ strikes the surface AC, it bends from optically denser to a rarer medium and follows path OR. This is the emergent ray.
4. When the ray of light enters the glass, it bends towards normal and when the ray comes out, it bends away from the normal. Now the angle between the emergent ray and incident ray is the angle of deviation. At the minimum deviation, The refracted ray inside the prism becomes parallel to its base.

Explanation:

1. When the refracted ray in the prism is parallel to its base deviation is minimum.
2. When the angle of incidence is equal to the angle of the emergence then the angle of deviation is minimum.
3. In a prism when the ray of light gets refracted minimum deviation is defined as the position at which the angle of deviation in the prism is minimum.

This is expressed as

${\mathrm{D}}_{\mathrm{m}}=\mathrm{A}\left(\mathrm{\mu }-1\right)$

Here,

${\mathrm{D}}_{\mathrm{m}}$ is the minimum angle of deviation.

A is the angle of the prism.

$\mathrm{\mu }$is the refractive index of the material of the prism.