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Mass of a photon in motion is ______.

Asked in GSEB Board July 2016 · Photon energy, momentum and mass

• (1) h/λ
• (2) c/(hf)
• (3) hf
• (4) (hf)/(c²)

Step-by-step solution

Why the other options are wrong

• (1) h/λ is the photon's momentum p, not its mass. Mass follows from p = mc, which gives m = h/(cλ).
• (2) c/(hf) is the reciprocal of the momentum (hf)/c. Its unit is s kg⁻¹ m⁻¹, not kg, so it cannot be a mass.
• (3) hf is the photon's energy. To turn an energy into a mass you must divide by c², from E = mc².

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