To Measure the Focal Length of a Convex Lens

Apparatus

Convex lens, raybox with cross-threads, screen, metre-stick or optical bench.

Method

Set up apparatus as in diagram.

Adjust the positions of the lens and the screen until the image of the cross threads is in sharp focus on the screen.

Measure u, the distance from the cross-threads to the centre of the lens.

Measure v, the distance from the screen to the centre of the lens.

Repeat for different values of u and v.

Graph 1/u against 1/v. Draw the best straight line.

Use this to obtain a single value for 1/u and for 1/v.

Calculate a value for the focal length of the lens, f, using the formula

Sources of Error

• The error of parallax when reading u and v.
• The image may not be in sharp focus.

Solutions

• Ensure your line of sight is at right angles to the metre stick.
• Perform the experiment in a dark room.