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Question

An astronomical telescope has on objective of focal length 200 cm and an eye piece of focal length 4.0 cm. The telescope is focused to see an object 10 km from the objective. The final image is formed at infinity. Find the length of the tube and the angular magnification produced by the telescope.

A
196 cm,50
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B
204 cm,50
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C
206 cm,50
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D
200 cm,50
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Solution

The correct option is B 204 cm,50
Here, the object is at a large distance (10 km) compared to the focal length of the objective. Hence the rays coming from the object can be treated as parallel.

Length of the tube L=f0+fe=200+4=204 cm

Angular magnification m=f0fe=2004=50

The final image will be real inverted.

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