Question

A telescope consists of two thin lenses of focal lengths $0.3m$ and $3cm$ respectively. It is focused on moon which subtends on angle of ${0.5}^o$ at the objective. Then, the angle subtended at the eye by the final image will be:

• A. $5^o$
• B. ${0.25}^o$
• C. ${0.5}^o$
• D. ${0.35}^o$

Answer 1

Answer: (A)

$5^o$

Angular magnification of telescope is given by:

$m=-\frac{f_o}{f_e}$

$m=-\frac{0.3}{0.03}=-10$

Also, angular magnification is:

$m=\frac{{\theta }_i}{{\theta }_o}$

$-10=\frac{{\theta }_i}{{0.5}^o}$

${\theta }_i=-5^o$ ($-ve$ sign denotes inverted image)