A solar cell is illuminated with ultraviolet light of a certain frequency. Electrons are released from the surface of the solar cell material. When the ultraviolet source is replaced with a dim green light source, no electrons are released. Explain these observations in terms of photons and the photoelectric effect.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Light travels as discrete packets of energy called photons.
The energy of a photon is given by E=hf, so higher frequency light has higher energy.
A photon must supply at least the work function of the material to free an electron.
Ultraviolet light has a higher frequency than green light, so each UV photon has enough energy to exceed the work function and release electrons.
Green photons have insufficient energy (their frequency is below the threshold), so no electrons are released even if the light is dim or intense.
The energy of a photon is given by E=hf, so higher frequency light has higher energy.
A photon must supply at least the work function of the material to free an electron.
Ultraviolet light has a higher frequency than green light, so each UV photon has enough energy to exceed the work function and release electrons.
Green photons have insufficient energy (their frequency is below the threshold), so no electrons are released even if the light is dim or intense.
Examiner tips
- Mention photons and their energy equation E=hf
- Explain the work function threshold
- Compare UV and green frequencies
- State that intensity cannot compensate for insufficient photon energy
Common mistakes
- Confusing intensity with photon energy
- Saying electrons are released by any light regardless of frequency
- Using incorrect units or omitting the work function concept
Mark scheme (5 marks)
- Light travels as discrete packets of energy called photons
- The energy of a photon depends on its frequency (higher frequency = higher energy photon)
- A single photon must supply enough energy to release one electron (equal to or greater than the work function)
- Ultraviolet light has a higher frequency than green light, so each UV photon has more energy, which is sufficient to release electrons
- Green light photons have insufficient energy (below the threshold frequency/work function) so no electrons are released regardless of intensity
Key terms in this question
photon · photoelectric effect · frequency
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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