Concepts of Physics

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

Focal Length of a Concave Mirror

Objective

To find focal length of a concave mirror.

Introduction

The concave mirror converges rays parallel to its axis on the focus. The distance between the pole of the mirror and its focus is called focal length.

Apparatus

Concave mirror, scale

Procedure

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Let us find focal length of a concave mirror. Face the concave mirror towards the sun. Take a small piece of paper and fix it to the thin stick. Using the stick as a handle, hold the paper very close to the mirror. Now gradually take the paper away from the mirror, towards the sun. At all times hold the paper at a small angle to the mirror to allow sunlight to fall on the mirror.

After a certain distance you will see an oval spot on the paper. Adjust the position and the angle of the paper till you get a very small, almost round, bright spot. At this position measure the distance between the pole of the mirror and the paper. This distance is approximate focal length of the mirror. If you keep the paper there for some time, the paper will start burning because sunlight gets concentrated at one spot, producing a lot of heat.