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, the angle of refraction, the angle of emergence and interpret the result.


MATERIALS REQUIRED


Drawing board, drawing pins, three plane sheets of white paper a rectangular glass slab, geometry instruments and pins.


THEORY

 When a light ray incident on air to glass interface through a glass slab obliquely has the following characteristics:
1. When a light ray travels from air to glass, the ray bends towards the normal at the surface of the air-glass boundary.
 
2. When a light ray travels from glass to air, the angle of refraction is greater than the angle of incidence of glass-air interfaces as the ray of light bends away from the normal


3. If the angle of incidence is zero, i.e. the incident ray is normal to the interface, the ray of light continues to travel in the same direction after refraction.
4. The angle of emergence and the angle of incidence will be equal.

PROCEDURE


1. Fix a white paper sheet on a drawing board using pins or cello tape.


2. Place a rectangular glass slab in the middle of the paper part and marks its boundary ABCD with a pencil.


3. Remove the rectangular glass slab and label the boundary as A1, B1, C1, D1 as shown below.


4. Draw normal (perpendicular) MN on the side AB at a point O, slightly away from the centre towards A1.


5. Draw an oblique line P1, Q1 (incident ray) such that ∠P1Q1M = 30 (Angle of incidence). Fix two sharp point pin P1 and Q1 vertically erected on the line P1Q1 at a distance of 5-7 cm apart.


6. Place the glass slab again within the marked position. Observe the image of Look of pins (not their heads) P1 and Q1 from the other side. Fix other two pins R1 and S1 in such a way that R1, S1, and the image of P1, and Q1 lie on a straight line.


7. Now, remove the glass slab and pins. Mark all the prick of the pins. Join the points R1 and S1 and produce a line upto the edge C1D1. Let R1S1 meet C1D1 at O2. This will act as an emergent ray.


8. Draw a normal M1, N1, at O1. Join O1 and O. It will represent the path of ray inside the glass slab, i.e. refracted ray.


9. Measure the angle of emergence, i.e. ∠e = ∠N1O2S1 and angle of refraction, i.e. ∠r = N1O1O2.


10. Repeat the experiment for two more values of angles of incidence such as 45 and 60 on the other part of the paper and measure the angle of refraction and emergence accordingly and tabulate them.



OBSERVATION TABLE


S.No.

Angle of incidence


(∠i)

Angle of refraction


(∠r)

Angle of emergence


(∠e)

∠i - ∠e

1.


2.


3.

30


45


60

19.45


44.97


35.26

30


48


62

0


3


2

RESULT


1. law of refraction is verified, i.e. at the point of incidence the incident ray, the emergent ray and the normal to the air-glass interface, all lie in the plane of the paper.


2. The relation between the angle of incidence and the angle of emergence are obtained within the experimental limits.


3. From the observation, the emergent ray emerging out of the rectangular glass slab is parallel to the incident ray.


4. Emergent ray undergoes lateral displacement.



5. The angle of refraction increases with increase in the angle of incidence.


PRECAUTIONS


1. The glass slab should be perfectly rectangular with all its faces smooth.


2. Glass slab must be clean and free from scratches.


3. Thin lines should be drawn to obtain good accuracy.


4. The distance between the pins should be 5-7 cm.


5. The base of all the pins should be placed in a straight line.

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