Experiment to measure the focal length of a convex lens

Apparatus:convex lens, screen, metre stick, light source with cross threads. Method: Find a rough value for the focal length of the lens by finding the image of a distant object into a screen. The distance from the screen to the lens is then the rough value of focal length. Set up the apparatus as in the diagram keeping the distance from the screen to the lens greater than the rough value of focal length. Adjust the position of the screen until a sharp image if the cross threads is seen. Then measure the image distance v and the object distance u. Use the formula 1/f=1/u + 1/v to find f. Repeat and find an average.

Another method involves plotting 1/u against 1/v and the value of 1/f is where the graph cuts the x and y axis. Results Precautions Avoid parallax when measuring distances. Repeat and average. u/cm v/cm f/cm