Aim
To trace the path of the rays of light through a glass prism.
Theory
• 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
• angle of incidence ∠i, ∠PQN
• angle of refraction ∠r, ∠FEN,
• angle of deviation ∠D, ∠HGF and
• angle of prism ∠A, ∠BAC.
∠A + ∠D = ∠i + ∠e
Materials Required
A white sheet, soft board, thumb pins, 4-6 all pins, prism, pencil, scale, protractor, drawing board.
Procedure
1. Fix a white sheet on a drawing board using drawing pins.
2. 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.
3. Draw a thin line NEN normal (perpendicular) to face AB of the prism. Also draw a straight line
PE making an angle 60° as shown in figure.
4. Fix two pins on the line PE as shown in the figure, later mark these points of pins as P and Q.
5. Look at the images of the pins, fixed at P and Q, through the other face of the prism, i.e., AC.
6. 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.
7. Remove the pins and the glass prism.
8. Join and produce a line joining R and S, let this line meet the prism at point F.
9. 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.
10. Measure the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e
and ∠D
Observations
Angles Values
∟i
∟r
∟A
∟D
∟D= ∟i + ∟e - ∟A
Source of error:
Conclusion
1. 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.
2. 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.
Precautions
1. A sharp pencil should be used for drawing the boundary of the prism.
2. 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.
