Two convex lenses with power 2 dioptre are kept in contact with each other. The focal length of the combined lens system is

1. 0.10 m
2. 2 m
3. 4 m
4. 0.25 m

The power of combination of lenses is the algebraic sum of the powers of individual lenses.

P = P1 + P2 = 4 m

1/f = 1/f1 + 1/f2

f = 1/4 = 0.25 m

The correct option is D.