Question

A lens has 25 cm. what is the power of the length and what its is nature?

Answer 1

Power of the lens is always the reciprocal of the focal length of the lens.
Focal length of the lens = f = 25 cm = 0.25 m
So, power of the lens = 1 / f
P = 1 / 0.25 
P = 4 D
Since power of a lens is positive which shows that, the lens is a converging lens.
Hopefully it helps.
Thanks.

Answer 2

The power of a lens (P) is inversely proportional to the focal length (f) and it is measured as the reciprocal of the lens’s focal length. The focal length of a lens is usually measured in meters and the power of the lens is measured as Dioptre. A focal length of 1 meter gives 1 dioptre power of the lens.

P = 1 / f (in meters)

P = 1 / 25 cm

P = 1 / 0.25 m

P = + 4 D.

Since it is a positive focal length, the lens will be a convex lens or a converging lens and the power is written as ‘+’.