A Lamp And A Screen Are Set Up 100 Cm Apart And A Convex Lens Is Placed Between Them. The Two Positions Of The Lens Forming Real Images On The Screen Are 40 Cm Apart. What Is The Focal Length Of The Lens? 

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Question

A lamp and a screen are set up 100 cm apart and a convex lens is placed between them. The two positions of the lens forming real images on the screen are 40 cm apart. What is the focal length of the lens? 

Solution

Correct option is

21 cm

 

Where now   

                     

Therefore x + 40 + x = 100 or x = 30 cm   

For lens at L1, we have  

                          u = –30 cm and v = +70 cm. Thus  

                      

  

or                f = +21 cm.

SIMILAR QUESTIONS

Q1

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Q2

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Q3

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Q4

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Q8

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Q9

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Q10

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