Explain why increasing the frequency of light incident on a metal surface causes electrons to be emitted with greater kinetic energy, whereas increasing the intensity of light of the same frequency does not increase the kinetic energy of the emitted electrons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When light shines on a metal surface, electrons can be emitted. This is known as the photoelectric effect. Scientists found that the kinetic energy of emitted electrons depends on the frequency of the light used, not its intensity.
Model answer (5 marks)
Light consists of photons, each carrying energy E = hf. The energy of a photon is proportional to its frequency. When a photon strikes a metal surface it can transfer all its energy to a single electron. The electron must first overcome the work function Φ of the metal; any excess energy becomes kinetic energy:
K = hf – Φ.
Thus, increasing the frequency increases hf, raising the kinetic energy of the emitted electrons.
Intensity is the number of photons per unit area per unit time. Raising intensity increases the number of photons, but each photon still has the same energy hf. Therefore the energy transferred to each electron, and hence its kinetic energy, remains unchanged.
K = hf – Φ.
Thus, increasing the frequency increases hf, raising the kinetic energy of the emitted electrons.
Intensity is the number of photons per unit area per unit time. Raising intensity increases the number of photons, but each photon still has the same energy hf. Therefore the energy transferred to each electron, and hence its kinetic energy, remains unchanged.
Examiner tips
- Use the equation K = hf – Φ to link frequency to kinetic energy.
- Explain that intensity changes photon number, not individual photon energy.
- Show the proportionality of photon energy to frequency.
- Mention the work function as the energy threshold.
Common mistakes
- Confusing intensity with frequency; claiming higher intensity raises kinetic energy.
- Failing to state that each photon interacts with one electron.
- Using the wrong equation (e.g., K = I or K = hf).
Mark scheme (5 marks)
- Light is made up of photons (discrete packets/quanta of energy)
- The energy of a photon depends on its frequency (E = hf, or energy is proportional to frequency)
- A single photon interacts with a single electron, transferring all its energy to that electron
- Any energy above the work function (the minimum energy needed to release the electron from the surface) becomes kinetic energy of the emitted electron
- Increasing intensity increases the number of photons but not the energy of each photon, so each electron still receives the same energy and is emitted with the same kinetic energy
Key terms in this question
frequency · intensity · kinetic energy
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