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Q.
A telescope has focal length of objective and eye-piece as $200\, cm$ and $5 \,cm$ respectively. What is magnification of telescope?
JIPMERJIPMER 2008Ray Optics and Optical Instruments
Solution:
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 $=200 \,cm$
$f_{e}=$ focal length of eye-piece $=5\, cm$
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.