Question

A converging lens of focal length 30 cm is placed in contact with another converging lens of unknown focal length then possible value for focal length of combination is

Answer 1

Given that

Focal length of a convex lens,F1=20

Focal length of a concave lens,F2=25

power of a convex lens,P1=100/F1

=100/20

=+5D

now,

power of a concave lens,P2=100/25

=-4D

therefore,

i)power of combination,P3=P1+P2

=5+(-4)

=1

ii)focal length of combination=100/P3

=100/1

=100cm

therefore the ans of power of combination and focal length are 1 & 100 cm