A metal surface is illuminated with visible light of high intensity, but no electrons are emitted. When the same surface is illuminated with ultraviolet light of much lower intensity, electrons are emitted immediately. Explain why electrons are emitted with ultraviolet light but not with visible light, and why the emission is immediate even at low intensity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Light consists of photons, each carrying energy E=hf. The energy of a photon increases with its frequency; ultraviolet light has a higher frequency than visible light, so its photons have greater energy. An electron can absorb only one photon at a time, and the photon must supply at least the work function φ to free the electron. Ultraviolet photons have E>φ, so they can liberate electrons, whereas visible photons have E<φ and cannot, regardless of intensity. Because a single UV photon can transfer all its energy to one electron, the electron is emitted immediately once the photon is absorbed, giving no delay even at low intensity.
Examiner tips
- Use the equation E=hf to link frequency and energy
- State the work function condition E>φ
- Explain one‑photon absorption
- Mention immediacy due to instant energy transfer
Common mistakes
- Confusing intensity with photon energy
- Claiming visible light can emit electrons if intensity is high
- Saying electrons are emitted after a delay
Mark scheme (5 marks)
- Light is made up of photons (packets/quanta of energy)
- The energy of a photon depends on its frequency (higher frequency = higher energy photon)
- Each electron can only absorb one photon at a time, so the photon must have enough energy to overcome the work function
- Ultraviolet photons have enough energy to exceed the work function / visible light photons do not have enough energy to release electrons regardless of intensity
- Emission is immediate because a single ultraviolet photon can instantly transfer all its energy to one electron, so there is no delay
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Photon and particle properties questions
- A student investigates the emission of electrons from a zinc plate using ultravi…
- The photoelectric effect cannot be explained using a wave model of light. Explai…
- A scientist investigates the photoelectric effect using a clean caesium metal su…
- Explain why increasing the frequency of light incident on a metal surface causes…