Question

if a concave lens of focal length 20cm and concave lens of focal length 30 cm are kept in the contact with each other find first what will be the focal length of combination second what will be the power of combination third what will be the behaviour of the combination

Answer 1

total focal length will be 30cm

power of lens=1/f

f=30 cm

hence power of lens will be 1/30 i.e 0.33..

the behaviour will be that it will create a point size virtual image