The focal lengths of the objective and eye-piece of a telescope are respectively 100cm and 2cm. The moon subtends an angle of 0.5∘ at the eye. If it is looked through the telescope, the angle subtended by the moon’s image will be
A
100∘
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B
50∘
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C
25∘
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D
10∘
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Solution
The correct option is C25∘ f0=100cm fe=2cm
Angular magnification of telescope
m=θiθo=fofe where θo is the angle subtended by the object at the eye and θi is the angle subtended by the final image at the eye.
⇒−1002=θi0.50 ⇒θi=−250 where the minus sign indicates the direction in which we measure the angle.