Q 12-09-073JEE MainJEE Main 2025 (24 Jan, Shift 2)Easy
A photograph of a landscape is captured by a drone camera at a height of $18\ \text{km}$. The size of the camera film is $2\ \text{cm}\times2\ \text{cm}$ and the area of the landscape photographed is $400\ \text{km}^2$. The focal length of the lens in the drone camera is
Answer: (A) $1.8\ \text{cm}$
The landscape is $20\ \text{km}\times20\ \text{km}$ and its image is $2\ \text{cm}\times2\ \text{cm}$. For a distant object the image forms at the focal plane, so the magnification is
$$m = \frac{f}{u} = \frac{2\ \text{cm}}{20\ \text{km}} = 10^{-6}$$
$$f = 10^{-6}\times18\ \text{km} = 0.018\ \text{m} = 1.8\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics