Tracing The Path Of A Ray Of Light Passing Through A Rectangular Glass Slab For Different Angles Of Incidence. Measure The Angle Of Incidence, Angle Of Refraction, Angle Of Emergence And Interpret The Result

Refraction is the property of light in which it bends from one medium to another. The law of refraction is also known as Snell’s law which states that the ratio of the sine of the angle of incidence to the angle of refraction is equal to constant. Below is an experiment to trace the path of a light ray passing through the rectangular glass slab.

Aim

To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result.

Theory

What are laws of refraction?

Following are the laws of refraction:

  • The incident ray, the normal ray and the refracted ray, all lie in the same plane.
  • Snell’s law: The law states that the ratio of the sine of an angle of incidence to the sine of the angle of refraction is constant.
    \(\frac{sin\;i}{sin\;r}=consatnt\)

What is refraction of light?

Refraction of light is a property of light in which the light changes its path when it passes from one medium to the other.

What is lateral displacement?

Lateral displacement is defined as the perpendicular shift in the path of light when it emerges out from the refracting medium.

Materials Required

  1. A drawing board
  2. 4-6 all pins
  3. White sheet of paper
  4. Rectangular glass slab
  5. A protractor
  6. A scale
  7. A pencil
  8. Thumb pins

Procedure

  1. Fix a white sheet on the soft drawing board using thumb pins.
  2. Using a sharp pencil, draw the outline boundary of the glass slab, place it at the of the white paper.
  3. Let ABCD be the rectangular figure obtained by drawing.
  4. Mark point E on AB and draw a perpendicular EN and label it as a normal ray.
  5. Draw one angle of 30° with the help of protractor with EN. Fix pins at P and Q at 4-5 cm on the ray that is obtained by the angle.
  6. Place the glass slab on the rectangular figure ABCD.
  7. To fix R and S, see through the glass slab of side CD, such that when seen through the glass slab, all the pins P, Q, R and S should lie in a straight line.
  8. Draw a small circle around the pins P, Q, R and S and remove the pins.
  9. Remove the glass slab.
  10. Join points R and S such that it meets CD at point F. Draw a perpendicular N’M’ to CD at point F.
  11. Using a pencil, join the points E and F.
  12. Measure the angles formed at AB and CD, i.e, the incident angle, refracted angle and emergent angle.
  13. The lateral displacement is obtained by extending the ray PQ in a dotted line which is parallel ray to FRS.
  14. Measure the lateral displacement.
  15. Repeat the same procedure for angle 45° and 60°.

Ray Diagram

reflection of light through a rectangular glass slab

Observation Table

Sl.no Angle of incidence
∠i = ∠PEN
Angle of refraction
∠r = ∠MEF
Angle of emergence
∠e = ∠SFM’
∠i – ∠e
∠PEN – ∠SFM’
1 30° 28° 30° 0°
2 45° 43° 44.8° 0.2°
3 60° 56° 59.8° 0.2°

Due to human error the value of i – ∠e may not be equal to zero.

Conclusion

  1. The angle of incidence and the angle of emergence are almost equal.
  2. As the light is travelling from rarer to denser optical medium, the angle of refraction will be lesser than the angle of incidence.
  3. For different angles of incidence, the lateral displacement will remain the same.
  4. The light will bend towards the normal when it travels from an optically rarer medium to an optically denser medium.

Precautions

  1. The rectangular glass slab used should have perfectly smooth faces.
  2. The should be easily fixed on the board. So the drawing board should be soft.
  3. The angle of incidence should lie between 30° and 60°.
  4. All pins base should be in a straight line.
  5. The distance between the pins P and Q or the pins R and S, minimum 5cm gap should be maintained.
  6. Using a sharp pencil, draw thin lines.
  7. The quality of the protractor should be good.
  8. Placement of the protractor should be correct to get correct measurements.
  9. The perpendiculars should be drawn with care.

Sources Of error

  1. There shouldn’t be any air bubbles in the glass slab.
  2. The measurements done by the protractor should be accurate.

Viva Questions

Q1. How is the angle of incidence related to the angle of emergence?
Ans: The angle of incidence is equal to the angle of emergence or may differ with 1°.

Q2. What should be the angle of incidence for the above experiment?
Ans: The angle of incidence required for the above experiment is between 30° to 60°.

Q3. On which condition does the light does not deviate during the refraction of light?
Ans: When the light ray is perpendicular to the refracting surface, there is no deviation of the light ray.

Q4. Which property of light causes the rainbow formation?
Ans: The rainbow formation is caused by the refraction of light due to the presence of tiny water droplets in the atmosphere.

Q5. On which condition will the emergent ray and the incident ray be parallel?
Ans: When the angle of incidence is equal to the angle of emergent ray, the emergent ray and incident ray will be parallel.

Stay tuned with BYJU’S to learn more about other Physics related experiments.

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