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Object: 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.
Apparatus Required: A drawing board, a sheet of white paper, drawing pins, rectangular glass slab, pins, protractor and metre scale.
Theory: In Fig. 4, a light ray AO is incident on the surface PQ of a rectangular glass slab PQRS. On entering into glass, it bends towards the normal NOM at the point O and takes OB inside the glass slab as refracted ray. The refracted ray is incident on the surface SR of the slab and on emerging out of glass, it bends away from the normal N₁BM at the point B taking the path BC as emergent ray. ∠AON = i is the angle of incidence, ∠BOM = r is the angle of refraction and ∠e is the angle of emergence.
The emergent ray BC is parallel to the incident ray AO but it is not in line with it. Thus the angle of emergence is equal to the angle of incidence i.
Method
- Fix a white sheet of paper on the drawing board with the help of drawing pins. Place the rectangular glass slab on the paper and draw its boundary line PQRS as shown in Fig. 5.
- Remove the slab and choose a point O on the boundary line PQ nearly at its middle. Draw a normal NOM at the point O on line PQ.
- Draw a line AO making an angle i = 30° (say) with the normal NOM.
- Replace the glass slab on its boundary line PQRS.
- Fix two pins a and b nearly 5 cm apart vertically on board, on the line AO.
- Keeping the eye on other side (RS) of slab, close to the plane of the board, fix two more pins c and d such that the bottom of all the four pins a, b, c and d appear to be in one straight line as seen through the slab.
- Remove the pins one by one and mark the position of each pin.
- Remove the glass slab. Join the points c and d to meet boundary line RS at B and extend it to C. Then join the points O and B by a straight line OB. Extend the line AO up to a point D outside the slab. You will notice that BC is parallel to OD.
- Draw a normal N1BM1 at the point B on line RS.
- Measure and record the angles AON = i (angle of incidence), BOM = r (angle of refraction) and CBM1 = e (angle of emergence) with the help of protractor.
- Repeat the experiment for angle i = 45° and 60°.
Observations
| No. of observations | Angle of incidence i = ∠AON (in degree) |
Angle of refraction r = ∠BOM (in degree) |
Angle of emergence e = ∠CBM1 (in degree) |
|---|---|---|---|
| 1 | 30° | 20° | 30° |
| 2 | 45° | 29° | 45° |
| 3 | 60° | 35° | 60° |
Result
- The angle of refraction r is less than its angle of incidence i. This shows that a light ray on entering into the glass slab bends towards the normal.
- The angle of emergence e is equal to its angle of incidence i. This shows that the emergent ray is parallel to its incident ray.
Precautions
- All pins should stand vertical.
- The pins a and b should be at least 5 cm apart.
- The glass slab must be clean and transparent.
Viva-Voce
Q.1. What is meant by refraction of light?
Ans. The change in direction of the path of a ray, when it passes from one medium to another medium, is called refraction of light.
Q.2. How does the path of a ray change when it enters a glass slab?
Ans. When a ray enters from air to glass, it bends towards the normal.
Q.3. How does the path of ray change when it emerges out of glass slab?
Ans. When a ray emerges out from glass to air, it bends away from the normal.
Q.4. How are the angle of incidence i and the angle of emergence e related in your experiment?
Ans. The angle of emergence e is equal to the angle of incidence i.
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Image Credits: Freepik
