The failure of classical wave theory to explain observations on…

The idea

Classical wave theory predicts that light of any frequency can eject electrons from a metal, provided the light is intense enough or shines for long enough, because energy is absorbed continuously and can accumulate. Observations contradict this: below a threshold frequency no electrons are emitted, however intense the light or however long it shines. Above the threshold, emission is almost instantaneous, even at low intensity, and the maximum kinetic energy of emitted electrons depends on frequency, not intensity. Classical theory also predicts that brighter light should give electrons more kinetic energy, but experiments show that intensity increases the number of electrons per second instead. Einstein’s photon model, with one photon interacting with one electron, explains these observations.

What earns the marks

• States the classical prediction that continuous wave energy can accumulate until electrons escape, regardless of frequency.
• Identifies the observation of a threshold frequency below which no photoelectrons are emitted, which classical theory cannot explain.
• Explains that emission is instantaneous above the threshold, whereas classical theory predicts a measurable time delay at low intensity.
• States that maximum kinetic energy depends on frequency, not intensity, contradicting the classical wave prediction.
• Explains that increasing intensity increases the number of photoelectrons per second, not their maximum kinetic energy.
• Links the failure of classical theory to the need for a photon or quantum model in which one photon interacts with one electron.

Common mistakes and how to put them right

• Claiming that classical theory predicts a threshold frequency.

  Put it right: Classical wave theory predicts no threshold, because any frequency can supply energy if intensity or time is sufficient.

• Saying that brighter light gives photoelectrons more kinetic energy.

  Put it right: Intensity increases the rate of emission, while frequency determines maximum kinetic energy.

• Describing the time delay as always observed.

  Put it right: Classical theory predicts a delay at low intensity, but experiments show emission is essentially immediate above the threshold.

Exam tips

• Structure your answer as prediction versus observation versus explanation, so each failure of classical theory is clear.
• Use the words threshold frequency, intensity and maximum kinetic energy precisely, and avoid using them interchangeably.
• Refer explicitly to the photon model when explaining why the observations support quantisation.