Question

The focal length of a lens is $10\,cm$. What is the power of a lens in dioptre?
(A) 0.1$D$
(B) 10$D$
(C) 15$D$
(D) 20$D$

Answer 1

Hint:The focal length is the measurement of how much light can converge or diverge. A converging lens is a convex lens and a diverging lens is a concave lens.
Here we shall use the formula $p = \dfrac{1}{f}$, where $p$is the power of the lens and $f$is the focal length.

Complete step by step answer:
The lens’s power is not directly proportional to the focal length. Convex lenses have positive focal length, so they have positive power results and concave lenses have negative focal length, so they have negative power results. Power of a lens is calculated in dioptre.
A lens strength is its capacity to manipulate light- the greater the power the greater the light’s refraction.
The axis that passes through the centre of the lens is the principal axis. The focal point is located on the principal axis.
$1$ dioptre is the unit for power of a lens whose focal length is $1$ meter.
Given,
Focal length of a lens $ = 10\,cm = 0.1\,m$
Power of a lens=?
We know that,
$p = \dfrac{1}{f}$
$p = \dfrac{1}{{0.1}} = 10\,D$
Hence, the power of the lens is $10$ dioptre.

Note:The power of a lens is measured in dioptre if and only if the focal length is in meter since diopter is a S.I unit. So, we should pay attention while calculating the focal length that is in meters. If not then we should convert it to meter. Also we have to see if the lens mentioned is convex or concave since the power will be positive for a convex lens and negative for a concave lens. If nothing is mentioned then we have to take the lens as a convex lens.