A double convex lens forms a real image of an object on a fixed screen. Now the lens is given a constant velocity v 0 along its axis and away from the screen. For the purpose of forming the image al

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A double convex lens forms a real image of an object on a fixed screen. Now the lens is given a constant velocity v 0 ​ along its axis and away from the screen. For the purpose of forming the image always on the screen, the object is also required to be given an appropriate velocity.

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Ans (D) If size of image size of object ​ = n v 1 ​ − u 1 ​ = f 1 ​ Using sign convention and differentiating: − v 2 1 ​ d t d v ​ + u 2 1 ​ d t d u ​ = 0 ⇒ d t d u ​ = v 2 u 2 ​ ⋅ d t d v ​ ⇒ ( v ′ − v 0 ​ ) = − n 2 v 0 ​ ⇒ v ′ = ( 1 − n 2 ) v 0 ​

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