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Q.

A totally reflecting, small plane mirror placed horizontally faces a parallel beam of light as shown in the figure. The mass of the mirror is 20 g. Assume that there is no absorption in the lens and that 30% of the light emitted by the source goes through the lens. The power of the source (in MW) needed to support the weight of the mirror is _____

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Detailed Solution

As 30% light is incident and reflected by mirror, force due to reflection of light on mirror is

F=2×0.3Pc=0.6Pc

 To support the weight of mirror, we use 0.6Pc=mg P=mgc0.6=20×103×10×3×1080.6=100×106 watt =100MW

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A totally reflecting, small plane mirror placed horizontally faces a parallel beam of light as shown in the figure. The mass of the mirror is 20 g. Assume that there is no absorption in the lens and that 30% of the light emitted by the source goes through the lens. The power of the source (in MW) needed to support the weight of the mirror is _____