Question

A telescope has focal length of objective and eye-piece as $200cm$ and $5cm$ respectively. What is magnification of telescope?

• A. 40
• B. 80
• C. 50
• D. 0.01

Answer 1

Answer: (A)

In relaxed eye state of telescope or when final image is formed at infinity, the magnification of telescope is given by
$|M|=\frac{f_o}{f_e}$
where $f_o=$ focal length of objective $=200cm$
$f_e=$ focal length of eye-piece $=5cm$
Hence, $|M|=\frac{200}{5}=40$
Note: The aperture of the objective lens is kept large so that more and more rays coming from the heavenly body may enter the telescope and a bright image is formed by the objective lens. The aperture of the eye lens is kept comparatively small so that all the rays may enter the eye.