Lab#02
Law of refraction of light using Prism
Objective:
To trace the path of the rays of light through a glass prism.
Equipment:
Following are the list of materials required for this experiment:
A white sheet
Soft board
Thumb pins
4-6 all pins
Prism
Pencil
Scale
Protractor
Drawing board
Theory:
What is a prism?
A prism is defined as a polyhedron with a triangular base and three rectangular lateral surfaces. It is
used as an optical object to study the behaviour of white light when it is passed through it. The light
bends at various angles like an angle of incidence, angle of reflection, angle of refraction, and angle
of deviation.
What is the angle of deviation?
The angle of deviation is defined as the angle between the incident ray and the emerging ray.
Procedure:
1. Fix a white sheet on a drawing board using drawing pins.
2. Place the triangular prism resting on its triangular base. Using a pencil, draw the outline of
the prism.
3. Draw NEN normal to the face of the prism AB. Make an angle between 30° and 60° with the
normal.
4. On the line PE, fix two pins at a distance of 5cm from each other and mark these as P and
Q.
5. Look for the images of the pins at P and Q through the other face of the prism AC.
6. Fix two pins at R and S such that they appear as a straight line as that of the P and Q when
it is viewed from the AC face of the prism.
7. Remove the pins and the prism.
8. At point F, make the points R and S meet by extending them.
9. PQE is the incident ray which is extended till it meets face AC. SRF is the emergent ray
10. Now mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e
which is extended backward to meet at point G.
and the angle of deviation ∠D as shown in the experimental setup.
11. Repeat the experiment for more angles between 30° and 60°.
Diagram:
Observation:
1. At surface AB, the light ray enters and bends towards the normal on refraction.
2. At surface AC, the light ray bends away from the normal as it travels from one medium
(glass) to the other (air).
3. The angle of deviation is observed. Here, the emergent ray bends at an angle from the
direction of the incident ray.
Conclusion:
1. The incident ray bends towards the normal when it enters the prism and while leaving the
prism it bends away from the normal.
2. With the increase in the angle of incidence, the angle of deviation decreases. After attaining
the minimum value, it increases with an increase in the angle of incidence.
Precautions:
For drawing the boundary of the prism, a sharp pencil should be used.
Soft board and pointed pins should be used.
The distance between the pins should be 5cm or more.
The pins should be fixed vertically and should be encircled when they are removed from the
board.
The angle of incidence should be between 30° and 60°.
The arrows drawn for incident ray, reflected ray and emergent ray should be proper.
For viewing the col-linearity of all four pins and images, the head should be slightly tilted on
either side. While doing this it can appear as if all are moving together.
