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A screen is kept at a distance of 1 m from the object. A converging lens between the object and screen when placed at any positions which are 60cm apart forms a sharp image of the object on the screen, then focal length of the lens is:

Option: 1

8cm


Option: 2

12cm


Option: 3

14cm


Option: 4

16cm


Answers (1)

best_answer

d= Distance between two positions of a convex lens.

D= Distance between object and screen

 

 f=\frac{D^{2}-d^{2}}{4D}

where D is the distance between the object and the screen and d is the distance between the two positions of lenses for which sharp images of the given object are obtained on the screen.

Here        D=1m=100cm        d=60cm

f=\frac{100^{2}-60^{2}}{4\times 100}=16cm

 

 

Posted by

Divya Prakash Singh

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