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Question

# The diameter of the moon is 3.5×103km and its distance from the earth is 3.8×105km. If it is seen through a telescope whose focal length for objective and eye lens are 4 m and 10 cm respectively, then the angle subtended by the image of the moon on the eye will be approximately

A
15
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B
20
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C
30
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D
35
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Solution

## The correct option is B 20∘The angle subtended by the moon on the objective of telescope α=3.5×1033.8×105=3.53.8×10−2rad. Let, the angle subtended by the image of the moon on the eye be β. Also, the angle subtended by the moon on the earth, as well as the eye will be very nearly the same. Therefore m=f0fe=βα⇒40010=βα⇒β=40α⇒β=40×3.5×1033.8×105×180π=20∘ (approx).

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