Question

If power of lens is -5 dioptre find its focal length?

Answer 1

Power= 1/f

Here power = -5D

F=1/5 m

F=0.2 m or 20 cm

Answer 2

Formula :

Power = 1 / focal length .

Given :

power = - 5 D

- 5 D = 1 / f

= > f = 1 / - 5

= > f = - 0.2 m

Now 0.2 m = 0.2 × 100

                   = 20 cm

So the focal length is 20 cm.

The lens has to be a concave lens since the focal length is negative.