Page 110 - Physics - XII

4. Now, move the source of light to 8 cm from the LDR and note the resistance. Repeat step 3 by moving
the source of light to 12 cm, 16 cm, and 20 cm from LDR and note the resistance.

5. Note all the observations in the observation table.

Observations
Table for variation in resistance of LDR with distance

S. No. Distance of light source from LDR (cm) Resistance of LDR (Ω)
1. 4

2. 8
3. 12

4. 16
5. 20

Result
As distance of light source from LDR increases, the resistance of LDR also increases.

Precautions

1. All the connections should be made tight.
2. LDR should be placed normally to the light source.

ACTIVITY - 4
Aim

To observe refraction and lateral deviation of a beam of light incident obliquely on a glass slab.

Apparatus and Materials Required
A rectangular glass slab, a drawing board, a sheet of white paper, drawing pins, offi ce pins, a protractor, a
metre scale, and a pencil

Principle/Theory

When a ray of light incident on a rectangular glass slab, it emerges out of it parallel to the direction
of incident ray. It only suff ers a lateral displacement which is proportional to the thickness of the slab.
(Fig. 4.1).

Procedure
1. Fix a white paper sheet on a drawing board with the help of drawing pins.
2. Take a glass slab and put it symmetrically in the middle of the white sheet paper and mark its boundary
ABCD.
3. Draw a normal N N at point X on face AB and draw a line PQ which makes an angle i with the
1
2
N N . Line PQ is incident ray.
1 2
4. Fix two offi ce pins at P and Q, about 8 to 10 cm apart, vertically on line PQ.
5. See images of these pins P and Q through the face DC and fi x two pins at points R and S such that the

pins R, S and the image of the pins P, Q lie in the same straight line.

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