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KCET2014PhysicsRay Optics

An object is placed at ( 20 ~cm ) in front of a concave mirror produces three times magnified real image. What is focal length of the concave mirror?

Options

  1. A( 15 ~cm )
  2. B( 6.6 ~cm )
  3. C( 10 ~cm )
  4. D( 7.5 ~cm )

Correct answer

A. ( 15 ~cm )

Step-by-step solution

Given, position of object ( =u=20 ~cm ); image formed ( =3 ) times magnified and real Using ( m= -v m ), we get ( 3= -v 20 v=-60 ~cm ) Using mirror formula, ( 1 f = 1 v + 1 w ), we get ( 1 f = 1 -60 + 1 -20 = -1-3 60 = -4 60 = -1 15 ) Therefore, focal length of concave mirror ( =15 ~cm )

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