First slide

Ray optics

Question

An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and the eye-piece is 36 cm and the final image is formed at infinity. The focal length $f_{0}$ of the objective and $f_{e}$ of the eye-piece are :

Moderate

A

$f_{0} = 45 c m a n d f_{e} = - 9 c m$
B

$f_{0} = 50 c m a n d f_{e} = 10 c m$
C

$f_{0} = 7.2 c m a n d f_{e} = 5 c m$
D

$f_{0} = 30 c m a n d f_{e} = 6 c m$

Solution

$f_{0} + f_{e} = 36$
$\frac{f_{0}}{f_{e}} = 5$
$f_{0} = 5 e$
$5 f e + f e = 36$
$f e = 6$
$f_{0} = 30$

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