A convex lens of power 4 D is combined with a other lens. For an object placed 25 m from the combination, the real image is formed 50 cm away from the combination. Find power of the other lens.

Explanation

Power of convex lens `=P_(1)=4D`
`u=-25cm`
`v=+50cm`
By lens formula we have
`1/v-1/u=1/f`
`1.50-1/(-25)=1/f`
`1/f=1/50+1/25=(1+2)/50=3/50`
`impliesf=50/3=1666cm=0.1667`m
Power of combination of lenses =P
`=1/(f("inm"))` M
`P=1/0.1667=6D`
Let `P_(2)=` Power of the other lens
`:.P=P_(1)+P_(2)`
`6=4+P_(2)`
`P=6-4=2D`