What Is Meant By The Power Of A Lens? Write Its Si Unit. A Student Uses A Lens Of Focal Length 40 Cm And Another Of 20 Cm. Write The Nature And Power Of The Lens

Answer: 

The power of the lens is the reciprocal of focal length. The SI unit is dioptre. 

Power of lens = 1/ focal length in metre.

F1 (focal length of first lens) = 40 cm = 0.4 m

Thus, the nature of this lens is concave lens. This can be ascertained from the positive value of the focal length. 

P1 = 1/f1

P1 = 1/0.4

P1 = 2.5 D

F2  (focal length of second lens) = -20 cm = – 0.2 m

Thus, the nature of the second lens is convex lens. This can be ascertained from the negative value of focal length. 

P2 = 1/f2

P2 = 1/(-0.2)

P2 = -5 D