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  4. The focal length of the objective of an astronomical telescope is 1.0 m. If the magnifying power of telescope is 20, then what is the length of the telescope for relaxed eye ?

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Q. The focal length of the objective of an astronomical telescope is 1.0 m. If the magnifying power of telescope is 20, then what is the length of the telescope for relaxed eye ?

Ray Optics and Optical Instruments

Solution:

$f_{0}=1 \,m$
$m=20$
for relaxed eye final image is formed at infinity.
$m=\frac{f_{0}}{f e}$
$20=\frac{1}{f_{e}}$
$f e=\frac{1}{20}=5 \,cm$
length of telescope
$L= f_0+f_e$
$100+5$
$105\,cm$