Explain why increasing the frequency of light shone onto a metal surface increases the maximum kinetic energy of the electrons emitted, but increasing the intensity of the same frequency light does not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A metal surface emits electrons when light above a certain frequency is shone onto it. This is known as the photoelectric effect.
Model answer (5 marks)
Light consists of photons, each carrying an energy E=hf where f is the frequency. A photon can only transfer its energy to one electron. If the photon energy exceeds the metal’s work function φ, the excess energy becomes the electron’s kinetic energy: KE_max=hf−φ. Therefore, increasing the frequency increases hf and so increases KE_max.
Intensity is the number of photons per second. Raising intensity adds more photons, but each photon still has the same energy hf. Thus each electron still receives the same maximum energy, and KE_max remains unchanged.
Intensity is the number of photons per second. Raising intensity adds more photons, but each photon still has the same energy hf. Thus each electron still receives the same maximum energy, and KE_max remains unchanged.
Examiner tips
- Use the equation E=hf to link frequency to photon energy
- Explain the one‑to‑one photon–electron interaction
- Show the work‑function subtraction to give KE_max
- Mention that intensity changes photon count, not energy per photon
Common mistakes
- Confusing intensity with frequency, claiming higher intensity gives higher KE
- Saying photons transfer energy to many electrons instead of one
- Forgetting to subtract the work function from hf
Mark scheme (5 marks)
- Light consists of photons (packets/quanta of energy)
- The energy of a photon depends on (is proportional to) its frequency
- A single photon interacts with a single electron; greater photon energy means more energy transferred to the electron
- After the work function energy is used to free the electron, any remaining energy becomes kinetic energy of the electron, so higher frequency means greater maximum kinetic energy
- Increasing intensity means more photons per second but each photon still has the same energy, so each electron still receives the same energy and maximum kinetic energy is unchanged
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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