A student shines a beam of red light onto a metal surface. No electrons are emitted. The student then replaces the red light with ultraviolet radiation of the same intensity. This time, electrons are emitted from the surface. Explain why electrons are emitted when ultraviolet radiation is used but not when red light is used.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Light travels as photons. The energy of a photon is E=hf, where h is Planck’s constant and f is the frequency. Ultraviolet light has a higher frequency than red light, so its photons carry more energy. An electron can absorb only one photon at a time. To escape the metal surface the absorbed energy must exceed the work function (threshold energy). Red‑light photons do not have enough energy to meet this threshold, so no electrons are emitted. Ultraviolet photons have sufficient energy, so electrons are emitted.
Examiner tips
- Use the formula E=hf to link frequency and photon energy
- Explain the work function and the one‑photon absorption rule
Common mistakes
- Confusing wavelength with energy; stating that all photons are absorbed by electrons
- Claiming that intensity, not frequency, determines electron emission
Mark scheme (5 marks)
- Light (electromagnetic radiation) travels as packets of energy called photons
- The energy of a photon depends on the frequency of the radiation
- Ultraviolet radiation has a higher frequency than red light, so ultraviolet photons have more energy
- Each electron can only absorb one photon at a time, so the electron needs a single photon with enough energy to escape the surface (the threshold/work function energy)
- Red light photons do not have enough energy to release an electron, but ultraviolet photons do have sufficient energy to free an electron from the surface
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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