Explain why the photoelectric effect cannot be explained by the wave model of light, and how the photon model successfully accounts for the existence of a threshold frequency.

AQA A-Level Physics (7408) — 3.2.2 EM Radiation and Quantum Phenomena · Explain · 5 marks · View as Markdown

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

When ultraviolet radiation is shone onto a zinc plate, electrons are emitted immediately. However, visible light of any intensity, no matter how long it is left shining on the zinc, fails to release any electrons at all.

Model answer (5 marks)

The wave model predicts that light energy is spread continuously over the wavefront, so any frequency of light should eventually supply enough energy to release an electron if the intensity or exposure time is increased. Consequently, the wave model would predict a gradual build‑up of energy and a delay before electrons are emitted. In contrast, the experiment shows that electrons are emitted immediately when ultraviolet light is shone on zinc, and that visible light, no matter how intense or how long it is applied, never releases electrons. This immediate emission cannot be explained by a continuous wave of energy.

In the photon model light is quantised into photons, each carrying a discrete energy of hf. A photon interacts with a single electron; if hf is less than the metal’s work function (ϕ), the electron cannot be released, regardless of how many photons (i.e. the intensity) strike the surface. Therefore a threshold frequency f₀ exists where hf₀=ϕ. Only photons with f≥f₀ have enough energy to liberate an electron, explaining why visible light (f<f₀) fails to produce photoelectrons even at high intensity.

Examiner tips

  • Use the word ‘immediate’ to show the instant emission; mention the delay predicted by the wave model.
  • Explain that intensity = number of photons, not energy per photon; emphasise the discrete energy hf.
  • State the threshold condition hf=ϕ to link frequency to work function.

Common mistakes

  • Confusing intensity with frequency; claiming higher intensity gives more energy per photon.
  • Suggesting the wave model predicts no delay; forgetting that energy builds up over time.
  • Failing to mention that a single photon can release only one electron, so many photons cannot compensate for insufficient energy per photon.

Mark scheme (5 marks)

  1. The wave model predicts that energy is spread continuously across the wavefront, so any frequency of light should eventually supply enough energy to release an electron given sufficient time or intensity.
  2. The immediate emission of electrons cannot be explained by the wave model, which would predict a delay as energy builds up.
  3. In the photon model, light is quantised into photons, each carrying a discrete packet of energy equal to hf.
  4. A single photon interacts with a single electron; if the photon's energy is less than the work function of the metal, the electron cannot be released regardless of how many photons (intensity) arrive.
  5. The threshold frequency is the minimum frequency at which a photon has exactly enough energy to equal the work function, so below this frequency emission is impossible no matter the intensity.

Key terms in this question

photoelectric effect · photon · threshold frequency · wave model

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