Refraction through Prism

 Prism is a solid piece of glass or transparent material that has at least two

planes inclined toward each other through which light is refracted.

Refraction of Light by rectangular glass Prism (Glass block)

• Let us consider a rectangular glass slab ABCD. When a light ray RQ after

travelling through air, is incident on the glass slab at a point P the light gets

refracted. Since the glass slab is a denser medium compared to air, thus the

ray gets bend towards the normal.

• The difference between glass slab and glass prism is that the opposite faces

of a rectangular glass slab is parallel while the opposite faces of prism is not

parallel to each other.

Triangular prism

• A transparent substance that can reflect light and has at least two lateral

surfaces that are obliquely inclined to one another. It contains five surfaces,

including three rectangle lateral surfaces and two triangle bases. The angle of

the prism refers to the angle created by two lateral surfaces. For a standard

prism, the prism’s angle is always 60°. See figs below

• Apex =The point at which two refracting surfaces of the prism meet is called

the refracting edge

• Rays leaving the prism is called emergent ray.

• Rays entering the prism is called incident ray

• Apical angle is the angle between the refracting surfaces (See the fig below)

Refraction of light through triangular glass prism

When a light ray is allowed to pass through a glass prism, it gets refracted twice.

First, at the



entrance of the prism and second when the light exits from the prism. Here the

two

refracting surface i.e. BO and CO are not parallel to each other. Thus the

incident ray is not parallel to the emergent ray. As it can be seen from the

diagram below a glass prism BOC is made to stand on its base BC. Here the

line ST and UV are normal to the surfaces BO and CO respectively. NW is the

incident light that is made to fall on the surface of prism that is, BO. As the light

ray is travelling from rarer medium to the denser medium the light ray bends

towards the normal ST and follows the path PR inside the prism.

PR is the refracted ray of light which bends towards the base of prism BC. Now

the



refracted ray PR after travelling through the prism, exits the prism at point R. At

this point

the ray is again refracted. Now since the refracted ray PR is travelling from

denser to rarer medium (glass to air) it bends away from the normal UV and

traverses the path RM.

This ray is known as Emergent Ray. In this case also we can see that the

emergent ray



bends towards the base of the

prism.

• Thus, we can conclude that when a ray of light travels through a prism, it

always tends to bend towards the thicker part of the prism.

• As we already know that both the surfaces of the prism are not parallel to each

other, therefore the emergent ray RM and incident ray NP are also not parallel

to each other. Now let us extend the incident ray NP to a point W. This

extended line is the original direction of the incident ray.

Angle of Deviation, D

• Is the angle formed by the intersection of the incident ray direction and the

emergent ray direction. (see the fig below)

• Now consider the diagram below;

From Snell’s Law