A student observes that a solar cell produces a larger electric current when bright sunlight shines on it compared to dim sunlight of the same colour. The student also notices that when red light shines on the solar cell, fewer electrons are released than when blue light of the same brightness shines on it. Explain both of these observations in terms of photons.

Eduqas A-Level Physics — 2.7 Photons · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Solar cells release electrons when light falls on them. The number of electrons released and the energy each electron receives depends on the nature of the light used.

Model answer (5 marks)

Light is made of photons. A brighter beam of the same colour contains more photons per second, so more electrons are knocked out of the cell each second, giving a larger current.

The energy of a photon is E=hf. Blue light has a higher frequency than red light, so each blue photon carries more energy. Electrons need a minimum energy to be released; blue photons can supply this energy more readily, so more electrons are released than from red photons of the same brightness.

Examiner tips

  • State that light is quantised into photons; link brightness to photon flux.
  • Use E=hf to explain why blue photons have higher energy than red.
  • Explain that higher photon energy means more electrons can be released.
  • Show the logical chain: more photons → more electrons → higher current.

Common mistakes

  • Confusing photon number with photon energy; saying brighter light has higher energy photons.
  • Using the wrong equation for photon energy (e.g., E=hc/λ without recognising λ is inversely related to frequency).
  • Failing to explain that current depends on the number of electrons released per second, not just the energy per electron.

Mark scheme (5 marks)

  1. Light consists of photons (discrete packets/quanta of energy)
  2. Brighter light of the same colour means more photons arriving per second
  3. More photons means more electrons are released per second, so a larger current is produced
  4. The energy of a photon depends on its frequency / blue light has a higher frequency than red light
  5. Blue photons have enough energy to release electrons (more easily / in greater numbers) whereas red photons have less energy so fewer electrons are released

Key terms in this question

photon · current

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