The photoelectric effect cannot be explained using a wave model of light. Explain how the photon model of light accounts for the key observations of the photoelectric effect.

Eduqas A-Level Physics — 3.2 Photon and particle properties · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When ultraviolet light is shone onto a clean zinc plate, electrons are emitted from the surface immediately, even at very low light intensities. If red light is used instead, no electrons are emitted regardless of the intensity.

Model answer (5 marks)

Light is made of discrete packets of energy called photons. Each photon can interact with only one electron, so the electron receives the full energy of that photon. The energy of a photon is E=hf; ultraviolet photons have a higher frequency and therefore higher energy than red photons. An electron can be emitted only if the photon energy exceeds the metal’s work function. If the photon energy is below the work function, no electron is emitted regardless of intensity. Because a single photon of sufficient energy can liberate an electron instantly, the emission is immediate.

Examiner tips

  • Use the phrase "one‑to‑one interaction" to show understanding of photon‑electron transfer.
  • Mention the equation E=hf to link frequency to energy.
  • Explain the work function as the minimum energy required.
  • State that emission is immediate because energy is delivered in one event.

Common mistakes

  • Confusing intensity with photon energy; students say higher intensity gives more energy.
  • Saying electrons need to absorb many photons; ignore the one‑to‑one rule.
  • Forgetting to mention the work function as the threshold energy.

Mark scheme (5 marks)

  1. Light (electromagnetic radiation) is made up of discrete packets of energy called photons
  2. Each photon interacts with only one electron (one-to-one interaction), so the electron receives all of one photon's energy in a single interaction
  3. The energy of a photon depends on its frequency (E = hf), so ultraviolet photons have higher energy than red photons
  4. There is a minimum energy (the work function) needed to release an electron from the metal surface; if the photon energy is below this, no emission occurs regardless of intensity
  5. Emission is immediate because a single photon of sufficient energy can release an electron instantly, with no need for energy to build up over time

Key terms in this question

photon · photoelectric effect

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