Question

(i) Draw a neat labelled ray diagram of an astronomial telescope in normal adjustment . Explain briefly its working .
(ii) An astronomical telescope uses two lenses of powers 10 D and 1 D . What is its magnifying power in normal adjustment ?

Answer 1

(i) Ray diagram of an astronomical telescope (normal adjustment) :
Working : Telescope has an objective and eyepiece . The objective has a large focal length and much larger aperture than the eyepiece . Light from a distant object enters the objective and a real image is formed in the tube at its second focal point . The eye piece magnifies this image and the final inverted image is formed at infinity .

(ii) The power of lens is 10 D
focal length = `(100)/(10) = 10` cm [`because P = (100)/(f("in cm"))]`
The power of lens is 1 D
`therefore " "` focal length = `(100)/(1) = 100` cm
Thus , `f_(0) = 100` cm `, f_(c) = 10` cm
Magnifying power = `(f_(0))/(f_(e)) = (100)/(10) = 10.`