Question

What is the unit of power of a lens?
(A) Watt
(B) horse power
(C) dioptre
(D) metre

Answer 1

Hint: Watt is the S.I unit of power which is $J{s}^{-1}$.
Horse power is also a unit of power but not of power of lens. It is the unit in foot-pound second(fps).
Dioptre is measurement of power of lens which is equal to $1m^{-1}$.
Metre(m) is the unit which is used to measure the distance or length.

Complete step-by-step answer:
Power of a lens is the ability of the lens to bend light. When the power of a lens increases the ability of the lens to refract the light also increases. In a converging lens, the converging ability is defined by power and in a diverging lens, the diverging ability is defined by power.
When the focal length increases the ability of a lens to bend the light decreases. So, we can come to the conclusion that power of a lens is inversely proportional to the focal length of the lens.
$\therefore P={\displaystyle \frac{1}{f}}$
where, $P$is the power of lens and $f$ is the focal length.
Watt is the S.I unit of power. Watt can be defined as the rate of consumption of energy of one joule per second.
$1W=1J{s}^{-1}$
Horse power(hp) is also a unit of power which can be defined as the rate at which work is done. 1 horsepower is equal to the 33,000 foot-pounds of work per minute.
Dioptre is the unit of power of a lens which is equal to the reciprocal of focal length which is measured in metres.
$\therefore 1D=1m^{-1}$
Focal length is measured in metres.
Metre(m) is the unit which is used to measure the length or distance.

Therefore, option (C) is the correct answer.

Note: Convex lenses always have focal lengths in positive sign therefore, the power of convex lenses is also positive.
Concave lenses have focal length in negative sign so, they have the power of lens in negative sign.