A screen is placed 90 cm away 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. Find the focal length of lens.
A
42.8 cm
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B
21.4 cm
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C
10.7 cm
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D
5.5 cm
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Solution
The correct option is B 21.4 cm
The image of the object can be located on the screen for two-position of convex lens such that u and v are exchanged.
The separation between two positions of the lens is, d= 20 cm