AS Daily A Level Physics question
A clean metal has a threshold frequency that lies between the frequencies of red and violet light. It is illuminated in two separate trials: Trial X uses low-intensity violet light; Trial Y uses high-intensity red light. Which statement must be true?
Answer
The correct answer is C.
Correct: C — Only Trial X emits photoelectrons; increasing intensity in Trial Y cannot make up for too low a photon frequency. A — Wrong: red light below threshold cannot eject any electrons, so both trials cannot emit. B — Wrong: brightness (intensity) increases the number of photons per second, not the energy per photon; with red below threshold, no electrons are emitted at all. C — This is the correct statement: frequency determines photon energy and thus whether emission occurs; intensity only affects emission rate once above threshold. D — Wrong: photoemission is not by gradual energy build-up; an electron is ejected only if a single photon has enough energy (above threshold).