Introduction

In this science experiment “Refraction of Light Through a Prism”, we shall learn to track the path of light rays through a prism.

Prism

A prism is a transparent, homogeneous glass enclosed by two inclined refracting surfaces at an angle called refracting angle. The refracting angle is also called the angle of prism.

It has:

1. Two triangular bases ABC and DEF,
2. Three rectangular lateral surfaces ADFC, ADEB and BCFE.
3. Six vertices A, B, C, D, E and F.
4. Nine edges AB, AC, BC, FD, DE, EF, AD, BE, and CF.

Important Points About Prism

1. Triangular prism is a refracting glass.
2. Angle of deviation is the angle between incident and ray and emergent ray.
3. Angle of deviation depends upon incident ray, angle of deviation and nature of prism.
4. Angle of incidence is inversely proportional to the angle of deviation, i.e. angle of incidence increases, the angle of deviation decreases.
5. The minimum angle of deviation of a prism is called minimum deviation.
6. Under this position (minimum deviation), refracted rays are parallel to the prism’s base.
7. The phenomenon of splitting white light into seven colours after refraction through a prism is called the dispersion of light.

8. In a glass prism, different colour shows different refractive indices due to the different speed of each colour inside the prism.
9. The seven colours in which the white light splits up are Violet, Indigo, Blue, Green, Yellow, Orange and Red. We call it VIBGYOR as a short form.
10. The Red colour has minimum angle deviation, and the violet colour has maximum angle deviation.
11. When two inverted prisms are placed together, we can again obtain white light.

Aim

To track the path of light rays through a prism.

Apparatus Required

1. A drawing board,
2. Drawing pins,
3. A prism,
4. Three white sheets,
5. Metre scale,
6. One pencil
7. One protector

Theory

Here, PQ = Incident ray
QR = Refracted ray
RS = Emergent ray
MM’= Normal
NN’= Normal
<i = angle of incidence
<r = angle of refraction
<e = angle of emergence
<A = angle of prism

1. On moving from denser medium to rarer medium, the light bends away from the normal while in moving from rarer to denser medium, it bends towards the normal.
2. The incident light ray PQ travel from a rarer medium to a denser medium (air to glass), it will bend towards the normal.
3. When the same light ray leaves the prism RS, it goes from a denser medium to a rarer medium. So it will bend away from normal.

Procedure

1. With the help of some drawing pins, fix a white sheet on the drawing board.
2. Place the prism in the middle of the paper.
3. Draw its boundary with a pencil. Name it ABC.
4. Remove the prism.
5. Extend the line AB to XY, as shown in the figure.
6. Draw a normal NN’ perpendicular to AB.
7. Draw an incident angle <EFN = 30°.
8. Again, place the prism on the drawn boundary.
9. Fix two pins Pand Q on points E and F.
10. Now, fix two pins R and S, where the images of pins P and Q are obtained.
11. Fix two more pins such that feet of P1 and Q1 appear on the same straight line as of R and S.
12. Remove all pins and prism. Mark every point.
13. Join P1 and Q1 at G on the side AB. HG is the emergent ray.
14. Repeat this experiment by taking different values of incident angle such as 30°, 45°, 50°.

Observations

1. When a light ray enters from air to glass, it bends toward the normal, and when it leaves the prism, i.e. moving from glass to air, it turns away from the normal.
2. Therefore, the light ray undergoes two deviations in its path.

Result

EFGH is the path taken by light rays.

Precautions

1. Prism should be thoroughly cleaned.
2. Pencil should be sharp for drawing boundaries.
3. Pins should be placed vertically.
4. The distance between two pins should be 6 cm.
5. The angle of incidence must be between 30° and 60°.
6. Arrows should be drawn to indicate the path of a light ray.

Sources Of Error

1. Pins may not lie on the same straight line.
2. Angle may not be taken accurately.
3. Pins can be bent.
4. Pins may not be pointed or sharped.

Additional Activity

To measure the angle of deviation.

1. Follow all the steps given above.
2. Draw a normal MM’ at G.
3. Produce EF to L and GH to O meeting at point J.
4. Measure <GJL as it is the angle of deviation.
5. Repeat this experiment by taking different values of incident angle such as 30°, 45°, 50°.
6. Draw a graph between the incident ray on the X-axis and the angle of deviation on the Y-axis.

Observation Table

Result

1. Firstly angle of deviation decreases with an increase in incident angle, but after a certain point, it will increase with the increase in incident angle.
2. From the above observation, the minimum value of angle of deviation =………..°.

Conclusion

In this way, we have learnt to track the path of light rays through a prism.

Viva Questions With Answers

Q.1 What was the aim of our experiment?
ANS.  To track the path of light rays through a prism.

Q.2 Describe prism?
ANS. A prism is a transparent, homogeneous glass enclosed by two inclined refracting surfaces at an angle called refracting angle. The refracting angle is also called the angle of prism.
It has-
1. Two triangular bases.
2. Three rectangular lateral surfaces.
3. Six vertices.
4. Nine edges.

Q.3 What do you mean by the angle of refraction?
ANS. The angle between the normal and the incident ray is called the angle of reflection.

Q.4 Name the scientist who obtained spectrum from light rays?
ANS. Sir Issac Newton.

Q.5 What do you mean by dispersion of light?

ANS. The phenomenon of splitting up white light into seven colours after refraction through a prism is called the dispersion of light.

Q.6 What is VIBGYOR?
ANS. Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Q.7 On what factors does the angle of deviation depends?
ANS.
1. Incident angle,
2. Angle of a prism,
3. Nature of prism.

Q.8 What is the angle of refraction?
ANS. The angle between the normal and the refracted ray is known as the angle of refraction.

Q.9 Name the property used by a prism to form a spectrum.
ANS. Refraction of light.

Q.10 How many times does the light faces deviation in its path in a prism?
ANS. Two

Vikram Sarabhai

An Indian physicist and astronomer.