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Refraction Through Prism

Table of Contents

Aim

To trace the path of the rays of light through a glass prism.

A prism has a triangular base and three triangular lateral surfaces. These surfaces are inclined to each other.

Refraction of light through a prism:

In the given figure, ABC represents the base of a glass prism. Let PE be the incident ray of light on face AB of the prism. EF represents the bending of light when it enters the prism and hence show the refraction of light.
RS is the emergent ray at face AC of the prism. The angle D shows the angle of deviation. The ∠BAC of the prism is called the angle of the prism and it is denoted by ‘A’.
In the figure, the relation between

A white sheet, soft board, thumb pins, 4-6 all pins, prism, pencil, scale, protractor, drawing board.

Fix a white sheet on a drawing board using drawing pins.
Place a glass prism on it in such a way that it rests on its triangular base. Trace the outline of the prism using a pencil.
Draw a thin line NEN normal (perpendicular) to face AB of the prism. Also draw a straight line PE making an angle preferably between 30° and 60° as shown in figure.
Fix two pins at a distance of 5 cm from each other on the line PE as shown in the figure, later mark these points of pins as P and Q.
Look at the images of the pins, fixed at P and Q, through the other face of the prism, i.e., AC.
Fix two more pins, at points R and S vertically such that the feet of pins at R and S appear to be on the same straight line as the feet of the images of the pins P and Q when viewed through the face AC of the prism.
Remove the pins and the glass prism.
Join and produce a line joining R and S, let this line meet the prism at point F.
Extend the direction of incident ray PQE till it meets the face AC. Also extend (backwards) the emergent ray SRF so that these two lines meet at a point G.
Mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e and ∠D as shown in the figure.
Repeat the experiment for more angle of incidence preferably between 30° and 60°.

The light ray enters the prism at surface AB, bends towards the normal on refraction.
At surface AC of the prism, this light ray bends away from the normal because it travels from a glass to air.
The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation (∠D).

The light ray, i.e., the incident ray first bends towards the normal when it gets refracted in the prism and while leaving the prism it bends away from the normal.
The angle of deviation first decreases with the increase in angle of incidence ∠i. It attains a minimum value then increases with further increase in angle of incidence.

A sharp pencil should be used for drawing the boundary of the prism.
Use soft board and pointed pins.
The pins should be fixed at a distance of 5 cm or more.
The pins should be fixed vertically and immediately encircled after they are removed.
While viewing the col-linearity of pins and images, the eye should be kept at a distance from the pins so that all of them can be seen simultaneously. The col-linearity of all the four pins can be confirmed by moving the head slightly to either side while viewing them. They all appear to move together.
The angle of incidence should be between 30° and 60°.
Proper arrows should be drawn for the incident ray, refracted ray and emergent ray.

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