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 cm × 2 cm and the area of the landscape photographed is 400 km². The focal length of the lens in the drone camera is: (a) 2.8 cm (b) 0.9 cm (c) 2.5 cm (d) 1.8 cm

Answer: (d). Landscape side = √(400) = 20 km is imaged on the 2 cm film side. m = (2 cm)/(20 km) = 10⁻⁶ For a far object the image is near the focus: f ≈ m u = 10⁻⁶ × 18 km = 1.8 cm So the answer is (d) 1.8 cm.

A photograph of a landscape is captured by a drone camera at a height… | JEE Main 2025 (24 Jan, Shift 2)

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Spherical Surfaces & Thin Lenses

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

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Answer: (d)

Landscape side $= \sqrt{400} = 20$ km is imaged on the 2 cm film side.
$m = \frac{2\text{ cm}}{20\text{ km}} = 10^{-6}$
For a far object the image is near the focus: $f \approx m u = 10^{-6} \times 18 \text{ km} = 1.8$ cm
So the answer is (d) 1.8 cm.

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