Angle of Minimum Deviation

Our aim in this experiment is to determine the angle of minimum deviation for a given prism by plotting a graph between the angle of incidence and the angle of deviation.

First, let’s understand the term “Deviation”,

Deviation of light can be defined as divergence in the path of light because of a change in the medium of propagation of light.

In this experiment, we are studying the deviation of light passing through a prism.

When a ray of light passes through a prism, it undergoes refraction twice from two adjacent surfaces. The final ray coming out of the prism deviates with respect to the ray incident on the prism. The angle between the two refracting surfaces is called the angle of the Prism (A).

The angle between the emergent ray and the incident ray is known as the angle of deviation.

Let us consider a case of minimum deviation to understand our experiment better.

A ray (TS) is incident on surface AB of the Prism at an angle of incidence i. The dotted line 

N’PN represents the normal to surface AB. After refraction from point P, it deviates along PO at an angle of refraction r. 

The ray again incidents at angle r₁ on surface AC and emerges (ray QR) at an angle e to the surface. The angle formed between ray TS (produced forward) and ray QR(produced backwards) is the angle of deviation ?.

From geometry, we can write as:

                        r₁  + r = A    (A is the angle of the prism)

                        ? = (i – r) + ( e –  r₁)

                      ? = (i  +  e )  – (r +  r₁)   = i  +  e – A

At the position of the prism in minimum deviation ?_m, the light ray passes through the prism symmetrically, i.e. parallel to the base. So, at this position:

                                     i =   e,  which implies r = r₁ 

The advantage of putting the prism in a minimum deviation position is that the image formed is the brightest in this position. This way, we can easily find the minimum deviation angle for a prism.

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