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 = reciprocal of the focal length [in meters]

SI unit is D (Dioptre)


Ans ) a) we have f=40cm=0.4m
p=1/0.4=5/2=+2.5D
*convex lens


b) f= -20cm=0.2m
p= 1/0.2= -5D
*concave lens