Define power of a lens. What is its unit ? One student uses a lens of focal length `50 cm` and another of `- 50 cm`. What is the nature of the lens and its and power used by each of the them ?

Explanation

Power of lens is defined as the ability of a lens to bend the light rays. It is given by the reciprocal of focal length of the lens in meter. Its unit is dioptre.
Focal length = f = 50 cm, then P = 100/f = 100/50 = 2D, lens is convex.
If focal length = f = - 50 cm, then P=100/f = 100/ -50 = -2 D, lens is concave.