Question

power of a concave lens is 4.5 diopter find the focal length

Answer 1

The focal length is the reciprocal of power in metres.
According to this,
Focal length is equal to 22.22 cm.
As this is a Concave mirror, therefore focal length would be negative, that is, -22.22cm

Answer 2

Power = 1/F ( in meters)

4.5 = 1/F

F = 1/4.5 = 0.22 m

= 22cm...

As focal length of concave lens is negative

Therefore F = -22cm...