Explain why increasing the frequency of light shone onto a metal surface causes electrons to be emitted with greater kinetic energy, even when the intensity of the light is kept the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Light can cause electrons to be released from the surface of a metal. This is known as the photoelectric effect. Scientists found that the kinetic energy of the emitted electrons depends on the frequency of the light used, not its intensity.
Model answer (5 marks)
Light consists of photons, each carrying a discrete amount of energy.
The energy of a photon is given by E=hf, where h is Planck’s constant and f is the light’s frequency. Thus, a higher‑frequency photon has more energy.
In the photoelectric effect a single photon is absorbed by a single electron. Part of the photon’s energy is used to overcome the metal’s work function (the energy required to liberate the electron from the surface). The remaining energy appears as the electron’s kinetic energy.
Because the photon energy increases with frequency, the excess energy after the work function is larger for higher‑frequency light, so the emitted electrons have greater kinetic energy even if the light intensity (number of photons) is unchanged.
The energy of a photon is given by E=hf, where h is Planck’s constant and f is the light’s frequency. Thus, a higher‑frequency photon has more energy.
In the photoelectric effect a single photon is absorbed by a single electron. Part of the photon’s energy is used to overcome the metal’s work function (the energy required to liberate the electron from the surface). The remaining energy appears as the electron’s kinetic energy.
Because the photon energy increases with frequency, the excess energy after the work function is larger for higher‑frequency light, so the emitted electrons have greater kinetic energy even if the light intensity (number of photons) is unchanged.
Examiner tips
- State the photon energy equation E=hf and explain its dependence on frequency.
- Mention the one‑to‑one photon‑electron interaction and the work function.
- Show how the remaining energy becomes kinetic energy.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing intensity with frequency; not recognising that intensity relates to photon number, not energy per photon.
- Using the wrong equation (e.g., E=hc/λ) without explaining the frequency dependence.
- Failing to mention the work function or the one‑to‑one interaction.
Mark scheme (5 marks)
- Light is made up of photons (discrete packets/quanta of energy)
- The energy of a photon depends on (is proportional to) its frequency
- A single photon interacts with a single electron (one-to-one interaction)
- Some photon energy is used to free the electron from the surface (work function / threshold energy)
- The remaining/extra energy becomes the kinetic energy of the electron, so higher frequency photons give electrons greater kinetic energy
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 →
More Photons questions
- A scientist is investigating the photoelectric effect using a clean zinc metal s…
- A student is investigating the emission of light from a LED (light-emitting diod…
- A solar cell is illuminated with ultraviolet light of a certain frequency. Elect…
- A student shines a beam of red light onto a metal surface. No electrons are emit…