Question

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

Answer 1

Hint: We know that a convex lens is one in which light rays after refracting from the lens gets converged in a point. And the distance from the centre of convex lens to the point where light rays get converged is called focal length of a convex lens.

Complete answer:
Let us first understand how this focal length is measured. As, when parallel light rays come from infinity and get refracted through the convex lens and get converged to a point which is called focal point and then this distance when measured from centre of the convex lens is called focal length and its always positive for the convex lens.

Firstly, put the convex lens in a holder on a bench table and keep it in front of some distant object, we can say a window or a tree. So that light rays can easily come through it. Now, put a cardboard screen just behind the lens. Now, we will change the distance of this cardboard screen continually and we will find a point when all rays from the object will concentrate at a point on the cardboard screen. Measure this distance from the lens to the cardboard screen, this length will be the focal length of the convex lens.

Note: Remember, when placing the lens in the holder and keeping a cardboard screen behind the lens adjust both objects at same level horizontally and keep checking if rays of light passing through lens or not, if not adjust the lens in front of some object from where light rays can easily travel.