Question

Plane surface of a thin plano-convex lens is silvered. If a point object is placed on principal axis at a distance `60cm` from the lens and fial image is formed at a distance `30cm` from the lens on same side. What will be the distance of final imagge from the lens if the plane surface is not silvered.
A. `120cm`
B. `180cm`
C. `30cm`
D. `90cm`

Answer 1

Correct Answer - A
Focal length of silvered mirror
`1/f=f/(f_(m))-2/f`
`1/f=1/(oo)-2/f`
`t=-f/2`
`1/v+1/u=1/f`
`1/(-60)+1/(-30)=1/f`
`f=-20cm`
Focal length of unsilvered lens will be `40cm`
So, `1/V+1/30=1/40`
`1/v=1/40-1/60=(3-2)/120=+1/120`
`impliesv=120cm`