Q: Photographs of the ground are taken from an aircraft flying at an altitude of 3 km by a camera with a lens of focal length 50 cm. The size of the film in the camera is 18 cm × 18 cm. What is the area of the ground that can be photographed by this camera ?

(a) 720 m × 720 m^{2}

(b) 240 m × 240 m^{2}

(c) 1080 m × 1080 m^{2}

(d) 100 m × 100 m^{2}

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Ans: (c)

Sol: As ground is at very large distance from camera, therefore image of ground can be assumed to be formed at the focus of the camera lens.

v = 50 m = 0.5 m , u =-3 km = -3000 m

If x is length of ground photographed, then

$\displaystyle m = |\frac{h_i}{h_o}| = |\frac{v}{u}|$

$\displaystyle |\frac{0.18}{x}| = |\frac{0.5}{3000}|$

x = 1080 m

Area of ground photographed = 1080 m × 1080 m