Question

what is meant by power of a lens? write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. write the nature and power of each lens.

Answer 1

power of a lens is defined as the reciprocal of its focal length

the nature of the first lens is convex lens

p = 1/f

p=100/f  { sinse its given in cm }

p=100/40 = 2.5D

the nature of second lens is concave

p=100/-20

p=-5D

hope it helps