Question

Two thin lenses of power + 4D and – 2D are in contact. What is the focal length of the combination

Answer 1

When two lenses are combined, the total power is the net of the power of the individual lenses.

So the total power is 4 - 2 = +2D

The focal length is the reciprocal of the power.

Therefore the focal length is 1/2m which is 0.5m

Answer 2

Power of combination= sum of power of individial lens

Let power of combination be Pc

So Pc= 4D+(-2D)=4D-2D=2D
Therefore answer is 2d