A Cassegrain telescope uses two mirrors. Such a telescope is bult with the mirrors 20mm apart. If the radius of carvature of the large mirror is 220mm and the small mirror is 140 mm, where will the final image of an object at infinity be ?

Explanation

`f_(0) = (220)/(2) = 110` mm
Object at infinity forms an image at 110 cm. This image will act as a yirtual object for the smaller mirror .
Distance of virtual object from smaller mirror
u = 110 - 20 90 mm
`f_(e) = (140)/(2) = 70` mm
Using `(1)/(v) + (1)/(u) = (1)/(f_(s)) rArr (1)/(v) = (1)/(f_(s)) - (1)/(u) = (1)/(70) - (1)/(90) = (1)/(315) mm `
v = 315 mm
i.e., Final image is 315 mm from smaller mirror on the righ side