Question

An astronomical telescope consists of an objective of focal length 60 cm and an eye - piece of focal length 5 cm. It is focused on a distant object such that the rays emerging from the eye - lens are parallel. The object subtends an angle of $2^{\circ }$ at the objective. The angular width of the image is

• A. ${10}^{\circ }$
• B. ${24}^{\circ }$
• C. ${48}^{\circ }$
• D. ${54}^{\circ }$

Answer 1

The correct option is B ${24}^{\circ }$

$M=\frac{f_0}{f_e},M=\frac{\beta }{\alpha }$

$\therefore \beta =\frac{f_0}{f_e}\alpha =\left(\frac{60}{5}\right)2^{\circ }={24}^{\circ }$