Question

define power of a lens what is its unit? One student use 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 the them​

Answer 1

Power of a lens is defined as the reciprocal of its focal length.

Its unit is dioptre and is represented as 'D'

case 1:

f=50 cm

P=?

P=1/f in m (or) 100/f in cm

P=100/50=+2

P=+2D

Its a concave lens

case 2:

f=-50cm

P=100/f in cm

P=100/-50

P=-2D

Its a convex lens

Answer 2

Power of lens is defined as the ability of a lens to bend the light rays. It is given by the reciprocal of focal length of the lens in meter. Its unit is Diopter.

Focal length of lens used by first student is in positive, hence it is a convex lens. The lens of second student is a concave lens.

FIRST STUDENT

focal length = f= 50 cm,

P = 100/f

  = 100/50

  = 2D, lens is convex

SECOND STUDENT

focal length = f = -50 cm,

P = 100/f

  = 100/-50

  = -2D, lens is concave