Page 86 - Physics - XII

Apparatus and Materials Required

A convex lens, a plane mirror, a clamp stand, a spherometer, an optical needle, a plumb line, a metre
scale, and clean transparent liquid
Object needle
O I Image
of object
needle O
Object needle
O I Image
of object b′ a′
needle O Convex
lens
b a C Transparent
liquid

C Plane mirror

Plane mirror Convex lens Fig. 7.2: Determination of focal length of
combination of convex lens and
Fig. 7.1: Determination of focal the plano-concave lens (made
length of convex lens of transparent liquid)

Principle/Theory
If the space between the lens of focal length f and plane mirror is fi lled with transparent liquid (water), a
1
plano-concave lens of water will be formed. Focal length of the plano-concave lens f formed is given by
1 1 1
f F f 1

where F is the focal length of the combination of two lenses, i.e. convex lens and plano-concave lens
formed of water.
From lens maker’s formula

1 ( 1) 1 1

f R 1 R 2

Now, R = R = Radius of curvature of convex lens.
1
and R = ∞
2
1 1 R
So, 1
f R f
The radius of curvature R of the convex surface of the lens is given by

l 2 h
R
6 h 2
where l is the mean distance between the two legs of the spherometer and h is the height of the central
screw above the plane of outer legs.

84