Optics - Telescope

A simple telescope consists of two convex lenses, the objective and the eyepiece, which have a common focal point P, as shown in the figure above. If the focal length of the objective is 1.0 meter and the angular magnification of the telescope is 10, what is the optical path length between objective and eyepiece? 

A. 0.1 m
B. 0.9 m
C. 1.0 m
D. 1.1 m
E. 10 m

(GR9277 #22)

Solution:

Magnification of the telescope: M = f₀/ f_e

The optical path length between objective and eyepiece d =  f₀ +  f_e

f₀ = focal length of the objective
f_e = focal length of the eyepiece

Since M = 10,  f₀ = 1

f_e = f₀/M = 0.1

d =  f₀ +  f_e = 1 + 0.1 = 1.1 m

Answer: D