Question

Draw a labelled ray diagram to show the image formation in a refracting type astronomical telescope in the normal adjustment position. Write two drawbacks of refracting type telescopes.

Answer 1

Refracting Astronomical telescope :

• It consist of an objective lens of larger focal length f0 and large aperture also an eye lens of small aperture and focal length

• In normal adjustment of the telescope the final image is formed at Infinity .

• A parallel beam of light from an astronomical object at infinity is made to fall on objective lens of telescope.

• It forms a real inverted and diminished imageA'B' of object .

• The eye piece is so adjusted that A’B’ lies at principal focus of eye piece.  

Therefore highly  magnified final image is formed at infinity.

The final image is erect with respect to A’B’ but it is inverted with respect to object . This does not matter as astronomical objects are normally spherical

Limitations :

Refracting Telescope suffers from chromatic aberration uses and  large sized lenses.

It is difficult and expensive to make such large sized lenses.

Answer 2

Answer:

Explanation:In normal adjustment final image is at infinity ,to have large magnifying power , fo must be as large as practically possible while fe has to be kept small.The magnifying power is the minimum and its field of view is small.