PHYSICS · Single correct · A Level · Physics · Particles and radiation · Particles and radiation
Problem
In an experiment, increasing the intensity of red light on a clean metal surface produces no emission of electrons, but switching to violet light of much lower intensity immediately ejects electrons. Which statement best explains the existence of a threshold frequency in this experiment?
(a) Electrons can be emitted only if several low-energy photons strike the same electron at the same instant, so intensity rather than frequency sets the threshold.
(b) Each photon must individually have energy E=hf at least equal to the metal’s work function ϕ before any electron can be released.
(c) Below the threshold, photons lack sufficient momentum p=λh to give electrons the impulse needed to escape the surface.
(d) Higher-frequency light creates a larger electric-field amplitude that pulls electrons out of the metal even if the photon energy is unchanged.