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A source of light is located at double f...

A source of light is located at double focal length from a convergent lens. The focal length of the lens is `f=30cm.` At what distance from the lens should a flat mirror be placed, so that the rays reflected from the mirror are parallel after passing through the lens for the second time?

Text Solution

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Object is at a distance of `2f=60 cm` from the lens. Image `I_1` formed by lens,
should be at a distance `60 cm` from the lens. Now `I_2,` the image formed by plane mirror should
lie at focus or at a distance of `30 cm` from the lens. Hence, the mirror should be placed at
distance `45 cm` from the lens as shown in figure.
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