Glass Prism Experiment

Experiment: To draw a graph between the angle of incidence (i) and angle of deviation (δ) for a glass prism and to determine the refractive index of the glass of the prism using this graph.

When light travels from one medium to another in which its speed is different, the direction of travel of the light is changed, when light travels from the medium of lesser speed to the medium of greater speed, the light is bent away from the normal. If light travels from a medium of greater speed to one of lesser speed, the light is bent towards the normal. The ratio of the sine of the angle of incidence in vacuum (i) to the sine of the angle of refraction (r) in a substance is equal to the ratio of speed of light (v1) in the vacuum to the speed of light in (v2) in the substance.

sin i / sin r = v1/v2 = n

where the constant n is called the refractive index of the substance.

If a ray MN of light is incident on one surface of a prism ABC, the ray is bent at both the first and the second surface. The emergent ray RS is not parallel to the incident ray but is deviated by an amount that depends upon the refracting angle A of the prism the refractive index n of its material and also on the angle of incidence (i) at the first surface. As the angle of incidence is decreased from a large value, the angle of deviation decreases at first and then increases and is minimum when the ray passes through the prism symmetrically. The angle of deviation, δm, is then called the angle of minimum deviation. For this angle of minimum deviation δm , there is a simple relation between the refracting angle A, the angle of minimum deviation δm, and the refractive index n. The relation is

n = sin i(A+δm)/sin (A/2)

Material Required

Drawing board, white paper, prism, pins, pencil, scale, protractor, drawing pins

How To Perform Experiment

1. Fix a sheet of a white paper on the drawing board.

2. Draw a line AB representing a face of the given prism. At a point N on this line, draw normal KN and a line MN at angle z representing an incident ray. Do not keep i less than 30° as the ray may get totally reflected inside the prism.

3. Place the prism on the sheet so that its one face coincides with the line AB. Refracting edge A of the prism should be vertical.

4. Fix two pins P1 and P2 on the line MN. Looking into the prism from the opposite refracting surface AC, position your one-eye such that feet of P1 and P2 appear to be one behind the other. Now fix two pins P3 and P4 in line with P1 and P2 as viewed through the prism.

5. Remove the pins and mark their positions. Put a scale along side AC, remove the prism and then draw a long line representing surface AC. Draw line joining P3 and P4. Extend lines P2 P1 and P4 P3 so that they intersect at F. Measure the angle of incidence i (angle MNK), angle of deviation D (angle RFG) and angle of prism (angle BAG).

6. Repeat the experiment for at least five different angles of incidence between 30° and 60° at intervals of 5°


Common prisms are usually quite small with sides of 2.5 cm or 3 cm. So drawing the boundary of the prism and then measuring angle A does not lead to accurate value of A. Therefore, it is suggested that you draw a long line for faces AB and AC with a ruler and place the prism touching the ruler.