Explain what a photon is and why higher frequency light transfers more energy to a metal surface than lower frequency light of the same intensity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists studying the photoelectric effect found that ultraviolet light could eject electrons from a metal surface, but red light of the same intensity could not, no matter how long it shone on the surface.
Model answer (5 marks)
A photon is a packet (quantum) of electromagnetic energy.
The energy of a photon is given by E=hf, where h is Planck’s constant and f is the frequency of the light.
Each photon interacts with one electron – a one‑to‑one interaction.
If the photon energy is below the threshold (the minimum energy required to free an electron from the metal) no electrons are ejected, no matter how many photons are incident.
Higher frequency photons have enough energy to overcome this threshold, so they can eject electrons, whereas lower frequency photons cannot.
The energy of a photon is given by E=hf, where h is Planck’s constant and f is the frequency of the light.
Each photon interacts with one electron – a one‑to‑one interaction.
If the photon energy is below the threshold (the minimum energy required to free an electron from the metal) no electrons are ejected, no matter how many photons are incident.
Higher frequency photons have enough energy to overcome this threshold, so they can eject electrons, whereas lower frequency photons cannot.
Examiner tips
- Use the equation E=hf to link frequency to energy.
- Explain the one‑to‑one photon‑electron interaction.
- Mention the threshold energy for ejection.
- Show why intensity does not help if photon energy is too low.
Common mistakes
- Confusing intensity with frequency – writing that more intensity means more energy per photon.
- Using the wrong equation (e.g., E=hc/λ) without converting to frequency.
- Claiming that electrons are ejected if many photons hit the surface, ignoring the threshold energy.
Mark scheme (5 marks)
- A photon is a packet (quantum) of electromagnetic energy
- The energy of a photon depends on its frequency / higher frequency means higher energy photon
- Each photon interacts with one electron (one-to-one interaction)
- If the photon energy is below the threshold / minimum energy needed to free an electron, no electrons are ejected regardless of intensity
- Higher frequency photons have enough energy to eject electrons because each individual photon carries sufficient energy to overcome the energy holding the electron in the metal
Key terms in this question
photon · frequency · intensity
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