A telescope has an objective lens with a focal length of 50 cm. An astronomer wants his telescope to have a magnification of (minus) 20. What should be the focal length of his eyepiece?
Can you please draw relevant diagrams and explain the working out?
The answer should be 2.5cm.
The magnification of a telescope is given by the formula :-
m = -fo/fe(1+fe/D)
Where, the above formula is used when the telescope forms the image at the normal distance of vision. Here, D = 25 cm.
Second arrangement of the telescope has the image formation at infinity, where D = infinity.
m = -fo/fe
where, fo = focal length of the objective
fe = focal length of the eye-piece
The magnifying power of the telescope is equal to the ratio of the angle formed by final images at eye to the angle subtended by object at eye.
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