Question

(i) A giant refracting telescope has an objective lens of focal length 30 m. If an eye peice of focal length 1.0 cm is used, what is the angular magnification of the telescope?
(ii) If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is $3.48\times {10}^6$ and the radius of lunar orbit is $3.8\times {10}^8$

Answer 1

(i) Angular magnification of a refracting telescope=$-\frac{f_o}{f_e}=-\frac{30}{{10}^{-2}}=-3000$

(ii) If the diameter of the image is $d$,

angle subtended by image=$\frac{d}{f_o}=\frac{d}{30}$

Angle subtended by diameter of moon=$\frac{3.48\times {10}^6}{3.8\times {10}^8}$
Equating the two gives, $d=27.47cm$