InterviewSolution
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InterviewSolution
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A luminous point object is moving along the principal axis of a concave mirror of focal length 12cm towards it. When its distance from the mirror is 20cm its velocity is `4cm//s`. The velocity of the image in `cm//s` at that instant isA. `6`towards the mirrorB. `6` away form the mirrroC. `9`away form the mirrorD. `9`towards the mirror. |
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Answer» Correct Answer - C `(1)/(v)+(1)/(u)=(1)/(f) rArr(1)/(v)+(1)/(-20)=(1)/(-12) rArr v=-30cm` Differentiling equation`(1)/(v)+(1)/(u)=(1)/(f)`w,r.t.time `-(1)/(v^(2))(dv//dt)-(1)/(u^(2))(du//dt)=0` ` dv//dt=-((v)/(u))^(2)xxdu//dt rArr =-(30/(20))^(2) xx4=9cm//s` `dv//dt=9cm//s`a way from the mirror. |
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