Q. 204.2( 20 Votes )

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.

Answer :

Given:


Distance between the image (screen) and the object, D = 90 cm
Distance between two locations of the convex lens, d = 20 cm
Focal length of the lens = f
Focal length is related to d and D as:


f = …(1)


Where,


f = focal length


D = distance between image and the object.


d = Distance between two locations of the convex lens.


By putting the values in equation (1) we get,



f = 21.39cm
Therefore, the focal length of the convex lens is 21.39 cm.


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