The energy of a photon, whose momentum is 10 MeV/c, where c is the speed of light, is given by
- A.10 MeV
- B.100 MeV
- C.1 MeV
- D.0.1 MeV
Correct answer
Explanation
The correct answer is A, 10 MeV. For a photon, energy equals momentum multiplied by the speed of light, E = pc, so a momentum of 10 MeV/c gives an energy of exactly 10 MeV. The general relation from Einstein's special relativity is E squared = (pc) squared + (mc squared) squared. A photon has zero rest mass, so the second term vanishes and the relation reduces to E = pc. That is why particle physicists often write momentum in units of MeV/c: multiplying by c simply returns the energy in MeV. The same result can be reached through Planck's relation E = hf together with de Broglie's p = h divided by wavelength, since c equals frequency times wavelength. Option B is wrong because 100 MeV would need a momentum ten times larger, 100 MeV/c. Option C is wrong because 1 MeV corresponds to a momentum of only 1 MeV/c. Option D is wrong because 0.1 MeV would mean dividing by 100, which no relation for a photon requires. Exam tip: for a photon E = pc and E = hf; for a massive particle at rest E = mc squared.
Practice Questions
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