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JEE Main 202524 JanuaryEvening Shift
Exam
JEE Main
Year
2025
Date
24 January
Shift
Evening Shift
Subject
Physics
Chapter
Ray Optics

JEE Main 2025 · 24 January · Evening Shift · Physics · Ray Optics

A photograph of a landscape is captured by a drone camera at a...

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 cm2 \mathrm{~cm} \times 2 \mathrm{~cm} and the area of the landscape photographed is 400 km2400 \mathrm{~km}^2. The focal length of the lens in the drone camera is :

Options

  1. A1.8 cm
  2. B0.9 cm
  3. C2.8 cm
  4. D2.5 cm

Correct answer

A. 1.8 cm

Step-by-step solution

H=18kmSize\mathrm{H}=18 \mathrm{ km} Size of camera film=2 cm\mathrm{ cm} \times 2 cm\mathrm{ cm}Aimage =400km2x=20×103m=2×104my=2×10−2mxy=106=18Kmff=18×10−3m=18mmf=1.8cm\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}$$
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