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Wave Optics

Question

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If the telescope (described in ques. 34) is used to view a 100 m tall tower 3 km away, what is the height of the image of the tower formed by the objective lens?

Solution

Height of the building = 100 m
Distance between the building and telescope = 3000 m

∴

Angle subtended by 100 m tall tower at 3 km,

θ = \frac{100}{3 \times 1000} = \frac{1}{30} radian

Let, y be the height of the image formed by the objective,
then,

θ = \frac{y}{f_{o}} = \frac{y}{140}

\Rightarrow

\frac{1}{30} = \frac{y}{140}

i.e.,

y = 4.7 cm .

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