Question

find the power of a convex lens which form a real and inverted image of magnification -1 of an object placed at a distance of 20 cm from its optical centre

Answer 1

u = object distance
v = image distance
f = focal length
first find v from formula
m = v/ u
-1= v/20
v=-20
find f by lens formula
i .e = 1/v -1/u =1/ f
-1/20 -1/-20 = 1/ f
f= 1
power of lens ( in cm ) = 100/ f
100/ 1
= 100 Ans