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A beam carrying energy E is incident perpendicular to a perfectly reflecting surface. The momentum delivered to the surface is

1. Ec
2. 2E/c
3. E/c
4. E/c²

Correct answer: 2E/c

Solution

When a beam of energy E strikes a perfectly reflecting surface, it imparts momentum to the surface upon reflection. The momentum change is twice the momentum of the incident beam, which is given by E/c, leading to a total momentum of 2E/c.

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