Question

A converging lens has a focal length of 50cm. The power of the lens is-
A) +5D
B) +2D
C) +0.5D
D) -0.5D

Answer 1

Hint: Power of a lens is defined as the reciprocal of focal length taken in the meter. i.e.
Power of lens $ = P = \dfrac{1}{f}$ where $f$=focal length and should be taken in meters.

Complete step by step answer:
As the given lens is a converging lens i.e. a convex lens. A convex lens has a real focus and therefore the focal length of a converging lens is always taken as positive,
$f = + 50cm$
Converting the focal length into meters,
$f = \dfrac{{50}}{{100}} = + 0.5m$
Now power of a lens is given by-
$P = \dfrac{1}{f}$
Put the focal length in above equation,
$P = \dfrac{1}{{ + 0.5}} = + 2D$

$\therefore$ The power of lens is +2D. Hence option (D) is the correct answer.

Note:
Keep in mind that while calculating the power of the lens the focal length should be taken in the meter. The physical significance of the power of the lens is the ability of the lens to bend a light, it means more the power of the lens more is the bending ability of the lens.