Question

The unit of power ($P$) of a lens is______
(A). $cm$
(B). $mm$
(C). $dioptre$
(D). $Hertz$

Answer 1

The power of a lens tells us how focused an image formed by a lens is. It is the reciprocal of focal length. Focal length is the distance from the midpoint of the lens to the focus. All rays parallel to the optical axis after reflection from the mirror or refraction from the lens passes through the focus.

Complete answer:
The power of a lens is defined as the reciprocal of its focal length. The greater the power the more focused, the image is and the smaller the focal length.
Concave lens and convex lens are two types of lenses. The focal length of a concave lens is positive as its focal length is positive, while the focal length of a convex lens is negative as its focal length is negative. This means that converging lenses have positive power while diverging lenses have negative power.
Power can also be adjusted by using a combination of lenses. There are several combinations possible, for example- concavo-convex, convexo-concave, plano-convex, plano-concave etc.
The power of a plane surface is zero as its focal length is infinity
The SI unit of power is $dioptre$ ($D$).
The other options are-
$cm$- It is the CGS unit of length.
$mm$- it is also a unit used to measure small lengths.
$Hertz$- is the Si unit of frequency. Frequency is the number of oscillations or vibrations completed per second.

Therefore, the unit of power of a lens is $dioptre$. So, the correct option is (C).

Note:
By convention all lengths measured from right to left are taken as negative, while all distances measured from left to right are taken as positive. Converging lenses are those in which the rays converge at a point after refraction. Diverging lenses are those in which rays diverge from a point after diffraction.