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  4. An observer looks at a distant tree of height 10 m with a telescope of magnifying power of 20. To the observer the tree appears :

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Q. An observer looks at a distant tree of height $10\, m$ with a telescope of magnifying power of $20$. To the observer the tree appears :

JEE MainJEE Main 2016Ray Optics and Optical Instruments

Solution:

Magnification of a telescope is given by:
$M =\frac{\theta_{ i }}{\theta_{ o }}$
$20=\frac{ h / d _{ i }}{ h / d _{ o }}$
$20=\frac{ d _{0}}{ d _{ i }}$
Distance of image, $d _{ i }=\frac{ d _{ o }}{20}$