Tracing the Path of the Rays of Light Through a Glass Prism

A prism is a transparent optical object with two flat surfaces that have an angle between them. When the light enters the prism, there is a bending of light as there is a change in the speed of light. The bending of the light is dependent on the angle of incidence, normal, and refractive indices. There are four different types of angles involved in this experiment, and they are the angle of incidence, angle of emergence, angle of prism and angle of deviation. Below is an experiment to trace the path of a light ray through a glass prism.

Aim

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

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 prism formula?

Following is the formula of the angle of prism:

\(\begin{array}{l}\mu =\frac{sin\frac{A+\delta _{m}}{2}}{sin\frac{A}{2}}\end{array} \)

where,
µ is the refractive index.
A is the angle of the prism.
δm is the minimum deviation.

What Is the angle of deviation?

The angle of deviation is defined as the angle between the incident ray and the emerging ray.

Materials Required

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

Experimental Setup

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 which is extended backward to meet at point G.
10. Now mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e and the angle of deviation ∠D as shown in the experimental setup.
11. Repeat the experiment for more angles between 30° and 60°.

Observations

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.