Question 149 of 295

A photograph of a landscape is captured by a drone camera at a height of 18 km . The size of the camera film is $2 \mathrm{~cm} \times 2 \mathrm{~cm}$ and the area of the landscape photographed is $400 \mathrm{~km}^2$. The focal length of the lens in the drone camera is :

A1.8 cmB0.9 cmC2.8 cmD2.5 cm

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Answer: (A) 1.8 cm

$\mathrm{H}=18 \mathrm{~km}$

Size of camera film $=2 \mathrm{~cm} \times 2 \mathrm{~cm}$
$\begin{aligned}
& A_{\text {image }}=400 \mathrm{~km}^2 \\
& x=20 \times 10^3 \mathrm{~m}=2 \times 10^4 \mathrm{~m} \\
& y=2 \times 10^{-2} \mathrm{~m} \\
& \frac{x}{y}=10^6=\frac{18 \mathrm{Km}}{\mathrm{f}} \\
& \mathrm{f}=18 \times 10^{-3} \mathrm{~m}=18 \mathrm{~mm} \\
& \mathrm{f}=1.8 \mathrm{~cm}
\end{aligned}$