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A small telescope has an objective lens ...

A small telescope has an objective lens of focal length `150 cm` and and eye piece of focal length `5 cm`. If his telescope is used to view a `100 m` high tower `3 km` away, find the height of the final image when it is formed `25 cm` away from the eye piece.

Text Solution

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`tan alpha=100/3000=1/30` radian
`tan alpha=h/f_0`
`1/30=h/150`
`h=5 cm`
h height of image of tower
`m_e=(1+ alpha/f_e)=(1+25/5)=6`
`m_e=h'/h`
`h'=5 times 6=30 cm`
h height of final image
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