EXPERIMENT – 4

AIM:

To determine the focal length of the given convex lens by using a candle and a screen and to calculate the power of a lens.

APPARATUS:

A convex lens, candle, scale, match box and a screen.

FORMULAE :



(Students are requested to leave four lines here)



Where:
f = focal length
P = power of the lens in dioptres
u = object distance
v = image distance

PROCEDURE :

1. Set up the experiment as shown.
2. Determine the approximate focal length by focusing a distant object like a tree on the opposite wall. Let its value be f.
3.Mark points at a distance f and 2f on one side of the lens.
4. A candle is kept beyond 2f on this side.
5. A screen is kept on the other side and its position is adjusted to obtain a sharp, inverted and a diminished image on the screen.
6. The values of u and v are noted
7. The candle is shifted to a new position and the screen is once again adjusted to record the new values of u and v.
8. Six such observations are taken and the focal length is calculated for all these observations.
9. The mean value of f is calculated.

PRECAUTIONS:

1. The object, lens and the screen should should be held vertical.
2. The objects likely to catch fire must be kept away from the candle flame.

RESULT:

The focal length of the lens is found to be ____ cm and the power of the lens is calculated as _____ dioptres. The power is positive as the lens is a convex lens.