Question

Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

Answer 1

Power of a lens indicates the converging effect of the lens. It is the reciprocal of the focal length of the lens.

Mathematically, `"Power" ("P") = 1/"f"`

The unit of power is dioptre (D).

The nature and power of the lens in each case are:

When the focal length of the lens is 50 cm,

`("P") = 1/"f"`

`= 1/0.5`

= 2D

This lens has positive value of focal length. So, it is convex lens.

When the focal length of the lens is -50 cm,

`("P") = 1/"f"`

`= 1/(-0.5)`

= -2 D

This lens has negative value of focal length. So, it is concave lens.