A Cassegrainian telescope uses two mirrors as shown in Fig. Such a telescope is built with the mirrors `20 mm` apart. If the radius of curvature of large mirror is `220 mm` and the small mirror is `40 mm`, where will the final image of an object at infinity be ?

Explanation

The image formed by the larger (concave) mirror acts as virtual object for the smaller (convex) mirror. Parallel rays coming from the object at infinity will focus at a distance of 110 mm from the larger mirror. The distance of virtual object for the smaller mirror `= (110 -20) = 90mm`. The focal length of smaller mirror is 70 mm. Using the mirror formula, image is formed at 315 mm from the smaller mirror.