Answer: Zero rest mass but carries energy.
- A Equal to electron mass according to conventional understanding
- B Zero rest mass but carries energy
- C 9.1 x 10<sup>-31</sup> kg in routine practice
- D Same as proton overall in most cases
Correct answer: B. Zero rest mass but carries energy
Explanation: Photon has zero rest mass but carries energy E = hf and momentum p = h/lambda.
In the photoelectric setup, light striking the metal plate ejects electrons that cross the evacuated tube to the collector, producing a measurable current on the ammeter.
Concept context
Photoelectric effect, de Broglie waves, Bohr's model, atomic spectra.