Question

In a telescope, why is the focal length of the objective lens large and that of the eye-piece small?

• A. The distance between the objective lens and the eye-piece is the sum of their focal lengths, and since the eye-piece is of small focal length, the objective lens should be of greater focal length
• B. This is essential to ensure a large magnification
• C. Telescope is meant to see the distant objects, so objective lens has to be of greater focal length to gather more light
• D. The statement is true only for the astronomical telescope

Answer 1

The correct option is B This is essential to ensure a large magnification

Magnification is given by

=$\frac{fo}{fe}$

So the magnification is directly proportional to the focal length of the objective lens and inversely proportional to the focal length of the eye piece. Hence it ensures large magnification of the distant object.