IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# To Find Focal Length of a Concave Lens

## Objective

To measure the focal length of a concave lens.

## Apparatus

Arrangement to fix the lens, arrangement to fix the laser, 30 cm plastic scale, 5 ft tape.

## Procedure

You are given a Laser and you can assume that it sends a parallel beam of light.

If this light goes through the concave lens it will diverge. If you place the screen at two different distances from the lens and measure a linear dimension such as height or diameter or something else, you can get similar triangle properties, $\frac{r_2}{r_1}=\frac{IB}{IA}=\frac{\mathrm{IP+PB}}{\mathrm{IP+PA}}$ You need to measure $$r_2$$, $$r_1$$, PA and PB in the experiment. From this you can calculate IP which is the focal length. Do it as accurately as you can.