Question

A telescope has an objective lens of focal length 200cm and an eyepiece with focal length 2cm. It is used to see a 5m tall building at a distance of 2km. The height of the image of the building formed by the objective lens is

• A. 5 mm
• B. 10 mm
• C. 1 mm
• D. 2 mm

Answer 1

Answer: (A)

5 mm

We use the formula for lens and magnification,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

and $m=\frac{-v}{u}$ $\frac{I}{O}$

Substituting the values, the height of the image formed will be, $I=5mm$