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(D) The magnification $m$ of a camera lens is given by the ratio of the image size to the object size,which is also equal to the ratio of the image di

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$(720\; m 	imes 720\; m)$

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(D) The magnification $m$ of a camera lens is given by the ratio of the image size to the object size,which is also equal to the ratio of the image distance $v$ to the object distance $u$.For an object at a large distance,the image is formed at the focal plane,so $v = f = 0.5\; m$.The object distance $u = 2000\; m$.The linear magnification $m = \frac{v}{u} = \frac{f}{u} = \frac{0.5\; m}{2000\; m} = \frac{1}{4000}$.The linear dimensions of the ground covered are $L_{ground} = \frac{L_{film}}{m} = L_{film} 	imes 4000$.Given the film size is $18\; cm 	imes 18\; cm = 0.18\; m 	imes 0.18\; m$.Therefore,the ground dimensions are $(0.18 	imes 4000\; m) 	imes (0.18 	imes 4000\; m) = 720\; m 	imes 720\; m$.

Photographs of the ground are taken from an aircraft flying at an altitude of $2000\; m$ by a camera with a lens of focal length $50\; cm$. The size of the film in the camera is $18\; cm \times 18\; cm$. The area of the ground that can be photographed by the camera is

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