Question

a convex lens of power 3 diopter is held in contact with a concave lens of power minus 1 diopter parallel beam of light is made to fall on the combination at what distance from the combination will the beam get focused

Answer 1

Concave, 25cm

Convex, 50cm

Concave, 20cm

Convex, 100cm

Answer :

b.

Solution :

Power of combination =(6−4)D=2D

Power =100f(in cm)

2=100f (in cm)

or f(in cm)=50

Since the net power is positive, therefore the combination shall behave like a convex lens.