Question

The focal length of a concave lens is 2m. Calculate the power of the lens.

Answer 1

Hint: The power of a lens can be calculated by taking the reciprocal of the focal length of the given lens. The sign of the power of a lens depends on the sign of the focal length of the given lens.

Formula used:
The power of a lens is given as
$P = \dfrac{1}{f}{\text{ }}...{\text{(i)}}$
Here P represents the power of the lens while f is the focal length of the given lens.

Complete step-by-step answer:
The power of a lens can be defined as the ability of a lens to bend the light rays passing through the lens. It is mathematically equal to the reciprocal of the focal length of the given lens. Concave lenses have negative power while convex lenses have positive power.
We are given a concave lens. The focal length of this lens is given as
$f = - 2m$
The power of the lens can be calculated by taking the reciprocal of the focal length of the lens using formula given in equation (i) in the following way.
$P = \dfrac{1}{f} = \dfrac{1}{{ - 2}} = - 0.5D$
This is the required answer to the question that the power of the concave lens of focal length 2m is -0.5D.

Note: 1. The units of power of a lens are dioptres (D). One dioptre is equal to the inverse of the unit of length.
2. The focal length of a concave lens is negative while that of a convex lens is positive because we take distances on the left of the left to be negative and to the right of the lens to be positive. The focus of the concave lens is on the left side as we calculate it by using the left curved surface of the lens.