A student investigates the emission of electrons from a zinc plate using ultraviolet light. The student finds that when dim ultraviolet light is used, electrons are emitted instantly, but when bright red light is used, no electrons are emitted at all. Explain these observations using the photon model of light.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A photon is a discrete packet of energy.
A single photon interacts with a single electron – the interaction is one‑to‑one.
Ultraviolet photons have a high frequency, so each photon carries enough energy to exceed the zinc work function; therefore electrons are emitted instantly, independent of the light intensity.
Red photons have a lower frequency, so each photon carries less energy than the work function; even if the light is bright, the photons cannot provide enough energy to liberate an electron, so no electrons are emitted.
A single photon interacts with a single electron – the interaction is one‑to‑one.
Ultraviolet photons have a high frequency, so each photon carries enough energy to exceed the zinc work function; therefore electrons are emitted instantly, independent of the light intensity.
Red photons have a lower frequency, so each photon carries less energy than the work function; even if the light is bright, the photons cannot provide enough energy to liberate an electron, so no electrons are emitted.
Examiner tips
- Use the term ‘photon’ and ‘work function’ – these are key words.
- Explain the one‑to‑one photon‑electron interaction.
- Show that UV photons have sufficient energy, red photons do not.
- Mention that intensity only changes the number of photons, not their energy.
Common mistakes
- Confusing light intensity with photon energy.
- Saying electrons are emitted ‘slowly’ instead of ‘instantly’ for UV.
- Using ‘light’ instead of ‘photons’ when describing energy transfer.
Mark scheme (5 marks)
- A photon is a discrete packet (quantum) of energy
- A single photon interacts with a single electron (one-to-one interaction)
- Ultraviolet photons have sufficient energy to exceed the work function / release electrons immediately regardless of intensity
- Red light photons have lower frequency than ultraviolet photons, so each red photon has less energy
- Red photons have insufficient energy to overcome the work function, so no electrons are emitted no matter how bright the light is
Key terms in this question
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