A parallel beam of intensity I is incident normally on surface A, which absorbs 50% and reflects 50%. The reflected light falls on perfect reflector B and returns to A; this process repeats. Find the radiation pressure on A.
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Question

A parallel beam of intensity I is incident normally on surface A, which absorbs 50% and reflects 50%. The reflected light falls on perfect reflector B and returns to A; this process repeats. Find the radiation pressure on A.

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G1 Physics · Accepted Answer

At each hit on A, 50% is absorbed and 50% reflected. Summing the geometric series of momentum transfer over repeated returns from perfect reflector B gives total pressure 3I/c. Correct answer: D.