Question

A small telescope has an objective lens of focal length 140 cm and eye piece of focal length 5.0cm. The telescope is used to view a 100 m tall tower 3km away. The height of the image of the tower formed by objective lens is

• A. $\frac{14}{3}cm$
• B. $\frac{11}{3}cm$
• C. $\frac{17}{3}cm$
• D. $\frac{8}{3}cm$

Answer 1

The correct option is A $\frac{14}{3}cm$

The magnification of an objective lens is given by, $\alpha$,
where, $\alpha$ = $\frac{h_o}{u_o}$ = $\frac{h_i}{v_o}$

We assume that the image is formed near the focus and thus, $v=f$
Substituting the values,

$\frac{100}{3000}=\frac{h_i}{140}$

Hence, $h_i$ = $\frac{14}{3}$