Question

two thin convex lenses of focal lenth 30 cm and 15 cm are placed in contact .what will be power of the combination

Answer 1

P = P1 + P2
P1 = 1 / 0.15 = 20/3 D
P2= 1/0.3 = 10/3 D
P = 20/3 + 10/3 = 30/3 = 10 Dioptre is the total power of the lenses.

Answer 2

For first lens,

Power = 1/focal length in meter
= 1/0.3
= +10/3 D

For second lens,

Power = 1/focal length in meter
= 1/0.15
= +20/3 D

Power of combination of lenses,
Arithmetic sum of powers of both lenses
= (+10/3) + (+20/3)
= (+30/3)
= +10 D