Four lenses of focal lengths $\pm 10$ cm and $\pm 200$ cm are available for making a telescope. To p

Question

Four lenses of focal lengths $\pm 10$ cm and $\pm 200$ cm are available for making a telescope. To produce the largest possible magnification, the focal length of the objective lens should be

Professor PC Thomas Classes · Answer

(c) : The magnifying power of an astronomical telescope is given by $M = f_o / f_e$. To achieve the largest possible magnification, the focal length of the objective lens ($f_o$) must be as large as possible, and the focal length of the eyepiece ($f_e$) must be as small as possible. Since real images are required for standard astronomical telescopes, converging (convex) lenses with positive focal lengths are used. Therefore, the objective should be the $+200$ cm lens.