Question

a convex lens of power `+6` dioptre is placed in contact with a concave lens of power`-4` dioptre. What will be the nature and focal length of this combination?
A. Concave, 25cm
B. Convex, 50cm
C. Concave, 20cm
D. Convex, 100cm

Answer 1

Correct Answer - b.
Power of combination `=(6-4)D=2D`
Power `=(100)/(f("in cm"))`
`2=(100)/(f)` (in cm)
or `f("in cm")=50`
Since the net power is positive, therefore the combination shall behave like a convex lens.