Question

How could you find the focal length of a convex lens rapidly but approximately? 

Answer 1

To determine the focal length of a convex lens, place the convex lens in a holder and keep it in front of a distant object like a tree. A cardboard screen is put behind the lens. Now, change the distance of the screen from the convex lens until a clear, inverted image of the tree is formed on the screen. Measure the distance of the screen from the lens. This distance will be the focal length of the convex lens. Here, we have used the fact that the image of an object at a far distance is formed at the focus of a convex lens.