Question

Describe in brief how would you determine the approximate focal length of a convex lens.

Answer 1

The approximate focal length of a convex lens can be determined by using the principle that a beam of parallel rays incident from a distant object is converged in the focal plane of the lens. This can be found out using the method described below:
In an open space, against a white wall, place a meter scale horizontally with its 0 cm end touching the wall.

By moving the convex lens to and fro along the scale, focus a distant object on the wall. The image which forms on the wall is very near to the focus of the lens and the distance of the lens from the image is read directly by the meter scale. This gives the approximate focal length of the lens.