Question

The power of a concave lens of focal length 25 cm is

Answer 1

The focal length of a lens is usually measured in meters and the power of the lens is measured as Dioptre. Afocal length of 1 meter gives 1 dioptrepower of the lens. Since it is a positivefocal length, the lens will be a convex lens or a converging lens and thepower is written as '+'.

Answer 2

Answer:

The power of a concave lens of focal length 25 cm is $- 4\ diopter$

Explanation:

• The power of a lens is given by the reciprocal of its focal length.
• Let P be the power and f be the focal length then,

        $P = \frac{1}{f}$   ...(1)

• For converging lenses, the focal length will be positive and for diverging lenses, the focal length will be negative

• So for the concave lens,  the focal length is considered to be negative.

In the question, it is given that,

$f = -25\ cm =- 0.25\ m$

Using equation (1),

$P = \frac{1}{-0.25} = -4\ diopter$

so we get the power of the given lens = $- 4\ diopter$