Concepts of Physics

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

Measure Approximate Focal Length of a Convex Lens


To measure focal length of a convex lens.


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


a convex lens, a piece of paper (preferably black), scale



Take a convex lens. You can also use a magnifying glass. Face the lens towards the sun. Take a small block of wood and place it close to the lens such that the lens is between the sun and the block. Slowly move the lens away from the block. At one stage, a very small, bright image will be formed on the block. The distance between the lens and the block in this position is the focal length of the convex lens.

This method does not work for concave lenses. A concave lens forms a virtual image which cannot be captured on a screen such as your wooden block. Other methods are used to find its focal length.