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

Explanation

Image formed by the first lens ltbr. `(1)/(upsilon_(1))-(1)/(u)=(1)/(f_(1))`
`(1)/(upsilon_(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 `(15-5)cm=10cm` 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)/(upsilon_(3))-(1)/(10)=(1)/(-10) or upsilon_(2) oo`
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)/(upsilon_(3))-(1)/(u_(3))=(1)/(f_(3)) or (1)/(upsilon_(3))-(1)/(oo)+(1)/(30)`
`or v_(3)=30 cm`
The final image is formed 30 cm to the right of the third lens.