A Cassegrain telescope uses two mirrors as shown in Fig. 9.33. Such a telescope is built with the mirrors 20 mm apart. If the radius of curvature of the large mirror is 220 mm and the small mirror is 140 mm, where will the final image of an object at infinity be?

Open in App

Solution

The following figure shows a Cassegrain telescope consisting of a concave mirror and a convex mirror.

Distance between the objective mirror and the secondary mirror, d = 20 mm

Radius of curvature of the objective mirror, R₁ = 220 mm

Hence, focal length of the objective mirror,

Radius of curvature of the secondary mirror, R₁= 140 mm

Hence, focal length of the secondary mirror,

The image of an object placed at infinity, formed by the objective mirror, will act as a virtual object for the secondary mirror.

Hence, the virtual object distance for the secondary mirror,

Applying the mirror formula for the secondary mirror, we can calculate image distance (v)as:

Hence, the final image will be formed 315 mm away from the secondary mirror.

Suggest Corrections

Similar questions

Q. In a Cassegrain telescope, which of the following mirrors is used as the secondary mirror?

Q. A reflecting telescope has a large mirror for its objective with radius of curvature equal to 80 cm. The magnifying power of this telescope if eye piece used has a focal length of 1.6 cm is:

Q. An object is placed at a distance of

25 cm

from the convex mirror and a plane mirror is set such that the image formed by the two mirrors coincide. The plane mirror is placed at

20 cm

from the object. The radius of curvature of the convex mirror will be

An object is placed at a large distance in front of a concave mirror of radius of curvature 40 cm. The image will be formed in front of the mirror at a distance of : ( (a) 20 cm (b) 30 cm c) 40 cm (d) 50 cm

If an object is at infinity (very large distance) in front of a concave mirror, where is the image formed ?

Join BYJU'S Learning Program