Find the position of the image formed by the lens combination given in Fig.
.

Explanation

Image formed by the first lens
`1/v_(1)-1/u_(1)=1/f_(1)`
`1/v_(1)-1/(-30) = 1/10`
or `v_(1)=15cm`
The image formed by the first lens serves as the object for the second. This is at a distance of (15-6) cm =10 cm to the right of the second lens. Though the image is real, it serves as a virtual object for the second lens, which means that the rays appear to come from it for the second lens.
`1/v_(2)=1/10=1/-10`
or `v_(2)=infty`
The virtual image is formed at an infinite distance to the left of the second lens. This acts as an object for the third lens.
`1/v_(3)=1/u_(3)=1/f_(3)`
or `1/v_(3)=1/infty+1/30`
or `v_(3)=30cm`
The final image is formed 30 cm to the right of the third lens.