Question

1 D is equal to:
A. $1m$
B. $1cm$
C. $1{m^{ - 1}}$
D. $1c{m^{ - 1}}$

Answer 1

Hint dioptre, also used as D is a unit of measurement of the optical power of a lens or curved mirror. This is basically equal to the reciprocal of the focal length (measured in metres).

Complete Step by step solution
The power P of a lens is defined as,
$P = \dfrac{1}{f}$
Where, $f$ is focal length.
Since the focal length is measured in metre, therefore the S.I. The unit of power of a lens is obviously inverse of metre.
Mathematically, we can write
Unit of power $
   = \dfrac{1}{{metre}} \\
   = > {m^{ - 1}} \\
 $

This is also known as dioptre.

Note Actually, it is a measure of the amount of deviation of light ray produced by a lens, more the power of the lens, more is the deviation. The focal length of the converging lens is positive and that of a diverging lens is negative. Thus, the power of a converging lens is positive and that of a diverging lens is negative. The power of the plane glass plate is 0.
If the thin lenses in contact are placed then the power of combinations of lenses kept close to each other is equal to the sum of individual powers of each lens. We replace the combination by a single lens and such a lens is called the equivalent lens for the combination.