Question

what is mean by power of a lens?write its SI unit. student images of lens of focal length 40 centimetre and another of - 20 CM. write the nature and the power of each lens

Answer 1

Power of a lens is the reciprocal of the focal length(in m). It's S.I. unit is Dioptre(D).
P = 1/f (in m) or P = 100/f (in cm)
Lense 1:-
100/40 = 2.5D
Lense 2:-
100/-20 = -5D

Answer 2

it js the degree of the lens where the converging and diverging of light occur is power of a lens
p = 1/f
f in meter
-20 means concave -20/100 = -.2 m
40 means convex 40/100 = .4 m
1/-.2 = -5
1/.4 = 2.5