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A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

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21.4cm

29.4cm

25.4cm

30.4cm

answer is A.

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Detailed Solution

Given, distance between screen and object a=90cm

Distance between two locations of the lens d=20 cm

$f = \frac{a^{2} - d^{2}}{4 a} = \frac{{(90)}^{2} - {(20)}^{2}}{4 \times 90} = \frac{7700}{360} = 21.4 c m$

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