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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?

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Object is at distance of 2f = 60 cm from the lens. Image formed by lens `I_1` should be at a distance 60 cm from the lens.
Now `I_2` the image formed by plane mirror should lie at focus or 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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