Question

(a) define the power of lens. (b) find the power of a concave lens of focal length 2 m.

Answer 1

a) The power of a lens is defined as the reciprocal of its focal length. It is represented by P. The power of a lens is related to focal length as below, Power of a lens = 1/Focal Length in meters.

b) Thus, the power of this concave lens is, -0.5 dioptre.

Answer 2

A ) One dioptre of the power of lens whose focal length is 1 metre.

B) Power = 1/ Focal length

= - 1/2 = - 0.50 D

Additional information :

• The ability of a lens to converge or diverge light rays is called power of the lens.

• It is defined as the reciprocal of focal length.

• P = 1/ Focal length (in metre)

• Its SI unit is Dioptre.

• If f is expressed in metres then power is expressed in dioptres.

• Thus dioptre is the power of a lens whose focal length is 1 metre.