Page 125 - Physics - XII

Table for current and distance of 100 W lamp from LDR

S. No. Distance of 100 W lamp from LDR (cm) Current (mA) Resistance R of LDR (Ω)
1. 5

2. 10
3. 15

4. 20

5. 25

Result

1. With increase in intensity of light, the resistance of LDR decreases and hence current fl owing in the
circuit containing LDR increases.
2. When distance of lamp (of fi xed power) from LDR increases, the intensity of light decreases and
resistance of LDR increases, so that current fl owing in the circuit containing LDR decreases.

Precautions

1. The source of light must be adjusted so that light falls normally on LDR.
2. Source of light must radiate light continuously.
3. Select suitable range in multimeter.

PROJECT - 3

Aim
To fi nd the refractive indices of (a) water (b) oil (transparent) using a plane mirror, an equiconvex lens
(made from a glass of known refractive index) and an adjustable object needle.

Apparatus and Materials Required
A convex lens, a plane mirror, a clamp stand, a spherometer, an object needle, a plumb line, a metre scale,
water, and a transparent liquid (turpentine oil)
Object needle
Object needle O I Image
O I Image of object
of object needle O
needle O

b a b′ a′
Convex lens
C Water or oil

Plane mirror Convex lens C

Fig. 7.1: Determination of focal Plane mirror
length of convex lens Fig. 7.2: Determination of focal length of
combination of convex lens and the
plano-concave lens (made of water or oil)

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