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

When `f = 50 cm, P = 100/f = 100/50 = 2` dioptre , Lens is convex.
When `f = - 50 cm, P = 100/f = 100/(-50) = - 2` dioptre , Lens is concave.