A refracting telescope has a magnification M. If the focal length of the objective lens is doubled and the eyepiece focal length is halved, what is the new magnification?
A) $M / 2$
B) $2 M$
C) $M / 4$
D) $M$
E) $4 M$

Question

A refracting telescope has a magnification M. If the focal length of the objective lens is doubled and the eyepiece focal length is halved, what is the new magnification? 
A) $M / 2$
B) $2 M$
C) $M / 4$
D) $M$
E) $4 M$

Quizplus · Accepted Answer

The magnification $M$ of a telescope is given by the ratio of the focal length of the objective lens ($f_o$) to the focal length of the eyepiece ($f_e$). If $f_o$ is doubled and $f_e$ is halved, the new magnification is $2f_o / (1/2f_e) = 4(f_o / f_e) = 4M$.

A Refracting Telescope Has a Magnification M M/2M / 2M/2 B) 2M2 M2M

A Refracting Telescope Has a Magnification M $M / 2$ B) $2 M$