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 power used by each of the them ?

Explanation

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