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# 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,

$$\begin{array}{l}\mu\end{array}$$
is the refractive index
A is the angle of prism
$$\begin{array}{l}\delta _{m}\end{array}$$
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

## 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.

## Viva Voce

Q1. What is refractive index?
Ans: Refractive index is defined as the ratio between the sine of the angle of incident ray i in a vacuum to the sine of the angle of refraction r in a given medium. The mathematical representation of the refractive index is given as:

 $$\begin{array}{l}n=\frac{sin\;i}{sin\;r}\end{array}$$

Where,

• n is the refractive index
• i is the angle of the incidence
• r is the angle of the reflection

Q2. What is the unit of the refractive index?
Ans: Refractive index has no unit. This is because it is a ratio between the speed of light in vacuum and in a medium.

Q3. Name the atmospheric refraction that causes the splitting of white light.
Ans: Rainbow.

Q4. What is the dispersion of light?
Ans: The dispersion of light is defined as the phenomenon of splitting white light into its seven constituent colours when it is made to pass through a transparent medium.

Q5. List the factors on which the angle of deviation through a prism depends.
Ans: Following are the factors on which the angle of deviation through a prism depends:

• ∠A is called the angle of a prism
• Angle of incidence
• On the optical density of the material used in a prism.

Q6. What happens to the incident ray when it enters the prism?
Ans: When an incident ray enters the prism, it bends towards the normal as it changes its path and gets deviated.

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