Question

Find the power of a concave lens whose focal length is 50 cm

Answer 1

Power of a lens is always reciprocal to the focal length. So, a convex lens with 50cm focal length has a power of (1/0.5) Dioptre(D) or 2D. Since, a convex lens is a converging lens, the power of the convex lens is +2D

Convex (So Focal length is positive)

Given : f = +50 cm or +50 x 10^-2 m

To Find : P = ?

Formula : P = 1/f

P = 1/50 x 10^-2

P = 100/50

P = 2

Therefore P = 2 dioptre or 2 D

Answer 2

power = 1 ÷ focal length

in metres p = 1 ÷ f

in cm p = 100÷ f

f = 50 cm

p = 100 ÷ 50

p = 2D