Concepts of Physics

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.

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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.