Focal length of a convex lense in air is `20cm.` and of concave lens 40cm. Find the position of final image.

Explanation

Using the lens formula for convex lens,
`u=-30cm, f=+20cm`
`:. (1)/(v)-(1)/(-30)=(1.)/(+20)`
Solving this equation, we get `v=+60.` Therefore, the first image is `60cm` to the right of
convex lens or `20cm` to the right of concave lens. Again applying lens formula for concave lens,
`u=+20cm, f=-40cm`, we have
`1/v-(1)/(+20)=1/-40`
`v=+40cm`
So, the final image `I_2` is formed at `40cm` to the right of concave lens as shown below.