Page 126 - Physics - XII

Principle/Theory

If the space between the lens of focal length f and plane mirror is fi lled with water or oil, a plano-
1
convex lens of water or oil will be formed, then focal length of the plano-concave lens f formed of
water or oil 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 or oil.

From lens maker’s formula

1 ( 1 1

f 1) R 1 R 2

Now, R = R = Radius of curvature of convex lens.
1
and R = ∞
2
1 1 R
So, f R 1 f

The radius of curvature R of the convex surface of the lens is given by

l 2 h
R = +
6h 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.

Procedure
Determinati on of the focal length of the convex lens (f )
1
1. Estimate the rough focal length of the convex lens.
2. Take a plane mirror and clean it. Place it on the horizontal base of an iron stand.
3. Place the convex lens on the plane mirror.
4. Now, fi x the object needle in the clamp stand and hold it horizontally above the convex lens, so that

its tip lies vertically above the optical centre of the lens as shown in Fig. 3.1. Adjust the height of the
needle equal to rough focal length of the convex lens. By keeping the eye at a distance of about 30 cm
from the needle, look for the image of the needle formed by convex lens and plane mirror.
5. Adjust the position of the needle O so that there is no parallax between the tip of the needle O and its
image I as shown in Fig. 3.1.

6. Using a plumb line and metre scale, measure the distance of the tip of the needle from the centre of the
upper surface of the convex lens. Let it be ‘a’. Now remove the convex lens and measure the distance
ab
of tip of the needle from the plane mirror. Let it be ‘b’. Then, focal length of the convex lens f .
1
7. Repeat the steps 1 to 6 at least two times more and take observations. 2
8. Note all the observations in the observation table.

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