Question

Define power of a convex lens.

Answer 1

Hint: In this question, we have to mainly focus on a convex lens, which converges rays of light that are parallel to its principal axis (i.e. converges the incident rays towards the principal axis) which is relatively thick across the middle and thin at the lower and upper edges. The diopter is the unit of measure for the power of a lens.

Complete Step-by-Step solution:
Power of a convex lens: -
Convex lens power is defined as the ability of the lens to converge the rays of light which fall on it as shown in the below figure.
                                                    

It can mathematically be defined as ${\text{P = }}\dfrac{{\text{1}}}{{\text{f}}}$, where f is the focal length.
 S.I. unit of power is the Diopter.
It is represented by Capital letter 'P'.
As we know, power of convex lens is the inversely proportional of its focal length in meters.
and unit of focal length f is meter
Therefore, the unit of power ${\text{mete}}{{\text{r}}^{{\text{ - 1}}}}$(${{\text{m}}^{{\text{ - 1}}}}$).

Note- Convex lens having positive power as a convex lens has a positive focal length, similarly the power of a concave lens is negative as a concave lens has a negative focal length. It is because the focal length of a convex lens is positive.