Question

Why reflecting telescope is superior in comparison to refracting telescope? Write two reasons.
Magnifying power of a telescope is $8$. When it is adjusted for parallel rays the distance between eyepiece and objective lens is $18$cm. Determine the focal length of both the lenses.

Answer 1

Reflecting telescope is superior in comparison to refracting because:

(1) Reflecting telescope do not suffer as much from chromatic aberrations.

(2) Reflecting telescope are cheaper to make.

(3) Light reflects off the primary mirror rather than passing through it so only one side of the reflector needs to be perfect unlike a refracting lens.

Magnifying power of telescope $M=-\frac{f_0}{f_e}$

$-8\times f_e=f_0$

Distance between eyepiece and objective is $L=|f_0|+|f_e|$

$18=8f_e+f_e$

$f_e=\frac{18}{9}=2cm$

$f_0=-8\times 2=-16cm$