Introduction

In this science experiment, we will learn to trace the path of a light ray passing through a rectangular glass slab for different incident angles and try to measure the angle of incidence, angle of reflection and angle of emergence. Thus we will explore Reflection Through Glass Slab.

Reflection of Light

1. When a beam of light enters from one medium to another, its path has a slight deflection. This happening is called the reflection of light.
2. When a light ray passes from a rarer medium to a denser medium, light rays bend towards the normal.
3. When a light ray travels from a denser medium to a rarer medium, light bends away from the normal.

4. In refraction, when light travels from one medium to another. There is no change in frequency
5. Speed and wavelength will change in refraction.

6. Speed of light reduces in going from rarer medium to denser medium.

Laws of Reflection

1. Incident rays, normal and reflected rays, all lie on the same plane.
2. It follows Snell’s law. According to Snell’s law of refraction, the ratio of the sine of incidence and the sine of reflection is constant.

                                       Sin <i/ sin <r = constant
                                                               = n21

 Where n21 is the refractive index of the second medium concerning the first medium.

Refraction Through Glass Slab

1. When light enters a glass slab, it bends toward the normal.
2. The incident angle is greater than the angle of reflection in entering from air to glass slab.
3. The angle of reflection is greater than the angle of incidence when travelling from glass slab to air.
4. If the light ray falls normally on a glass slab, it will travel in the same direction without deflection.
5. The angle of emergence is equal to the angle of incidence.
6. The line of emergence is parallel to the line of incidence
7. Lateral displacement is proportional to the angle of incidence and the thickness of the glass slab.

Aim

To track the path of a light ray passing through a rectangular glass slab for different incident angles and try to measure the angle of incidence, angle of reflection, and angle of emergence.

Apparatus Required

1. Rectangular glass slab,
2. Drawing board,
3. Drawing pins,
4. Three plain sheets of white paper,
5. Geometry instruments.

Theory

1. The refraction will occur at both sides of the rectangular glass slab, i.e. air-glass interface and glass-air interface.
2. When a light ray travels from air to glass, it bends toward the normal.
3. When a light ray travels from glass to air, it bends away from the normal.
4. The angle of incidence and the angle of emergence are equal.
5. The ray which leaves the glass from another side is called an emergence ray.
6. The angle that the emergence ray makes with normal is known as the emergent angle.

Procedure

1. With the help of drawing pins, fix the drawing sheet on a drawing board.
2. Draw a vertical line dividing the sheet into two equal parts.
3. Draw the boundaries of the rectangular glass slab by placing it on the white sheet.
4. Name these boundaries as A, B, C and D.
5. Draw a normal N1 N2 from the side CD.
6. Draw an incident ray PQ such that <PQN1= 30°
7. Fix two pins on P and Q vertically erected at a distance of 4cm and 6cm from point O.
8. Place the glass slab again within this side of the boundary.
9. Now, look at the pins from the other side of the glass slab AB.
10. Fix the pins R and S on pin P and Q images on the other side of the paper. R and S must make a straight line P and Q.
11. Draw a normal M1M2 from O’.
12. Join O and O’. It will show the path of the ray inside the glass slab.
13. Measure the angle of emergent <M2O’R and angle of incident <N1OP.
14. Repeat this experiment by taking different incident angles such as 60° and 45° with different sides of the white sheet.

Measurements And Observations

Result

1. The incident ray, reflected ray, and normal all lie on the same plane of the paper.
2. You will find, emergent angle and incident angle are equal.
3. Emergent ray and incident ray are parallel.
4. When a light ray travels from air to glass, it bends toward the normal.
5. When a light ray travels from glass to air, it bends away from the normal.

Precautions

1. The drawing board should be made up of wood.
2. The glass slab should be thoroughly cleaned.
3. The angle of incidence must lie between 30° and 60°.
4. All the pins should be placed vertically and in a straight line.
5. Enough presence of light.

Conclusion

In this experiment, we have learned to trace the path of a light ray passing through a rectangular glass slab for different incident angles and try to measure the angle of incidence, angle of reflection, and the angle of emergence.

Viva Questions With Answers

Q.1 Describe refraction?
ANS. When a beam of light enters from one medium to another, there is a slight deflection in its path. This happening is called the reflection of light.

Q.2 What was the aim of our experiment?
ANS. To track the path of a light ray passing through a rectangular glass slab for different incident angles and try to measure the angle of incidence, angle of reflection, and angle of emergence.

Q.3 Does the reflection depend upon frequency?
ANS. No

Q.4 Which physical quantity depends on refraction?
ANS. Velocity and wavelength.

Q.5 What is the relation between the velocity of light and the refractive index?
ANS. Refractive index = Velocity of light in vacuum/Velocity of light in the medium.
n21 = c/v

Q.6 Name the factor of the medium on which the refractive index depends?
ANS. 1. Density of medium,
Nature of medium.

Q.7 Describe lateral displacement?
ANS. The perpendicular or normal distance between the incident ray and the emergent ray is called displacement.

Q.8 What is Snell’s Law?
ANS. According to Snell’s law of refraction, the ratio of the sine of incidence and the sine of reflection is constant.
Sin <i/ sin <r = constant
                         = n21

 Q.9 Name the conditions when no refraction takes place?
ANS. 1) When an incident ray falls normally on a glass slab.
2)  When both the mediums are the same.

Q.10 What are some natural phenomena based on refraction?
ANS.
1. Twinkling of a star,
2. Words appear to be nearer when viewed by glass slab,
3. The bottom of the pool appears to be raised.

Vikram Sarabhai

An Indian physicist and astronomer.