Question

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.ย 

A

42.8 cm

B

21.4 cm

C

10.7 cm

D

5.5 cm

Medium
Solution
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Correct option is B)

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
From the figure,
Solving,
From lens formula,
ย 



Hence, the correct option is

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