Question

Draw a labelled ray diagram of an image formed by a refracting telescope with final image formed at infinity. Derive an expression for its magnifying power with the final image at infinity.

Answer 1

The magnifying power m is the ratio of the angle p substended at the eye by the final usage to the angle a which the object subtends at the lens of the eye. Hence,

f_e = focal length of eye piece

f_o = focal length of objective In this case, length of the tube = f_o + f_e

Magnification will be length as f_o>> f_e