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

Q 9.20 A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by  20cm. Determine the focal length of the lens.

Answers (1)

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As we know that the relation between focal length f, the distance between screen D and distance between two locations of the object d is :

f=\frac{D^2-d^2}{4D}

Given:D = 90 cm.,d = 20 cm , 

so 

f=\frac{90^2-20^2}{4*90}

f=\frac{90^2-20^2}{4*90}=\frac{770}{36}=21.39cm

Hence the focal length of the convex lens is 21.39 cm.

Posted by

Pankaj Sanodiya

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