A Cassegrain telescope is built with mirrors 20 mm apart. If the radius of curvature of the large mirror is 220 mm and the small mirror is 140 mm, then where will the final image of an object at infinty be?

Explanation

The image formed by the primary mirror is the virtual object for the secondary mirror.
For the primary mirror, `u = oo, f = (220)/(2) = 110 mm`

`therefore " " v = f = 110 mm`
For the secondary mirror,
`f=(140)/(2)`
= 70 mm
and u = 110 - 20 = 90 mm
Using mirror formula, `(1)/(v) = (1)/(f) - (1)/(u)` we get,
`(1)/(v) = (1)/(70) - (1)/(9)`
`or, " " v = 315 mm`
`therefore` The image will be formed at a distance 315 mm on the right of the secondary mirror.