Explain why increasing the intensity of light does not cause photoelectric emission from a metal surface, but increasing the frequency of light above the threshold frequency does.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When light shines on a metal surface, electrons may be emitted. This is called the photoelectric effect. Scientists found that whether electrons are emitted depends on the frequency of the light, not its intensity.
Model answer (5 marks)
Light is made of photons, each photon carrying an energy E=hf. Only one photon can interact with one electron – a one‑to‑one interaction. Increasing the intensity of light increases the number of photons, but the energy of each photon remains the same. If the photon energy is below the metal’s work function, no electron can be released, regardless of how many photons arrive. When the frequency is increased above the threshold frequency, the photon energy hf exceeds the work function, so each photon can liberate an electron. Thus intensity does not affect emission, but frequency does.
Examiner tips
- Use the equation E=hf to link frequency to photon energy.
- Explain the one‑to‑one photon‑electron interaction.
- Mention the work function as the minimum required energy.
- Show that intensity changes photon number, not energy.
Common mistakes
- Confusing intensity with photon energy.
- Assuming all photons can release electrons regardless of frequency.
- Forgetting to mention the work function.
Mark scheme (5 marks)
- Light consists of photons (packets/quanta of energy)
- Each photon interacts with one electron only (one-to-one interaction)
- Increasing intensity increases the number of photons but not the energy of each photon
- Each photon must have enough energy to overcome the work function (minimum energy needed to release an electron)
- Higher frequency means higher energy photon (energy depends on frequency), so above the threshold frequency each photon has enough energy to release an electron
Key terms in this question
threshold frequency · intensity
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Photon and particle properties questions
- A physicist is studying the photoelectric effect using a clean caesium metal sur…
- A student shines ultraviolet light onto a negatively charged metal plate. The pl…
- A student is investigating the emission of electrons from a metal surface. They …
- A scientist is investigating the wave and particle properties of electrons. When…