First slide

The optical instruments

Question

A telescope has an objective lens of focal length 200 cm and an eye piece with focal length 2 cm. If this telescope is used to see a 50 meter tall building at a distance of 2 km, what is the height of the image of the building formed by the objective lens

Moderate

A

5 cm
B

10 cm
C

1 cm
D

2 cm

Solution

$Magnification of objective lens m = \frac{I}{O} = \frac{v_{0}}{u_{0}} = \frac{f_{0}}{u_{0}} \Rightarrow \frac{I}{50} = \frac{200 \times 10^{- 2}}{2 \times 10^{3}} \Rightarrow I = 5 \times 10^{- 2} m = 5 cm$

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