Question

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.

Answer #1

The magnification of a telescope is given by the formula :-

m = -f_{o}/f_{e}(1+f_{e}/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 = -f_{o}/f_{e}

where, f_{o} = focal length of the objective

f_{e} = 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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