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Ashok0210
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Asked: 2021-03-21T03:48:41+00:00 In:

A screen is placed 90 cm 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. Determine the focal length of the lens.

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Class 12 Physics
CBSE and UP Board
Ray Optics and Optical Instruments
Chapter-9 Exercise 9.20
NCERT Solutions for Class 12 Physics Chapter 9 Question-20

2020-2021cbse and up boardclass 12 chapter-9 physicsclass 12 physicsncert class 12 ray optics and optical instruments
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      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:

      f2 = (D2-d2)/4D

      = [(90)2 – (20)2]/ (4 x 90) = 770 /36 = 21.39 cm

      Therefore, the focal length of the convex lens is 21.39 cm

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