Question

What is meant by power of lens? Write its si unite. A student uses lens of focal length 40cm and another of -20cm. Write the nature and power of each lens?

Answer 1

power of lens ids defined as the reciprocal of focal length in metre.
SI unit is diophtreD

Answer 2

Answer:

The power of the lens is 2.5 D and this lens is convex lens. the power of the lens is -5 D and this lens is concave lens.

Explanation:

Given that

Focal length $f_{1}=40\ cm$

Focal length $f_{2}=-20\ cm$

Power of lens:

The power of lens is the reciprocal of the the focal length of the lens and its S.I unit is dioptre.

$P = \dfrac{1}{f}$

Therefore,

The power for $f_{1}$ is

$P=\dfrac{100}{40}$

$P=2.5\ D$

The nature of the lens will be convex lens because focal length is positive.

The power for $f_{2}$ is

$P=\dfrac{100}{-20}$

$P=-5\ D$

The nature of the lens will be concave lens because focal length is negative.

Hence, The power of the lens is 2.5 D and this lens is convex lens. the power of the lens is -5 D and this lens is concave lens.