InterviewSolution
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InterviewSolution
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A linear object AB is placed along the axis of a concave mirror. This object is moving towards the mirror with speed U. The speed of the image of the point A is 4U and the speed of the image of B is alSO 4U btu in opposite direction. If the center of the line AB is at a distance L from the mirror then find out the length of the object. A. `3L//2`B. `5L//3`C. LD. None of these |
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Answer» Correct Answer - c. For concave mirror, if x and y are object distance and image and distance, recpectivley, we have `-(1)/(x)-(1)/(y)=-(1)/(|f|)` `rArr(1)/(x)+(1)/(y)=(1)/(|f|)` `rArr-(1)/(x)(dx)/(dt)-(1)/(y)(dy)/(dt)=0` `rArr |(V_(x))/(V_(y))|=(x^(2))/(y^(2))` For`rArr |(V_(x))/(V_(y))|=(1)/(4),(x)/(y)=+-2` For `(x)/(y)=2` we get `x=(3|f|)/(2)` [for point A] and For`(x)/(y)=-2`, we get `x=(|f|)/(2)` [for point B] As the middle happens to be focus of the mirror, we get `|f|=L` |
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