Question

A simple telescope consisting of an objective of focal length 60 cm and single eye lens of focal length 5 cm is focused on a distant object in such a way that parallel rays emerge from the eye piece. If the object makes an angle of 2${}^{\circ }$ at the objective, the angular width of the image is:

• A. 10${}^{\circ }$
• B. 24${}^{\circ }$
• C. 50${}^{\circ }$
• D. 60${}^{\circ }$

Answer 1

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

In case of a simple microscope,

$\frac{\beta }{\alpha }$ = $\frac{f_o}{f_e}$
where,
$\beta$ is the angular width of the image.
$\alpha$ is the angle subtended at the object.
$f_o$ is the focal length of the objective
$f_e$ is the focal length of the eyepiece

Substituting the values,
$\frac{\beta }{2}=\frac{60}{5}$

Hence, $\beta$ = ${24}^0$