A student shines monochromatic light onto a metal surface and observes that electrons are emitted. The student then replaces the light source with one that emits light of the same intensity but a lower frequency. No electrons are emitted. Explain these observations in terms of photons and the work function of the metal.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Light consists of photons, each carrying an energy E=hf. The metal has a work function φ, the minimum energy required to liberate an electron. With the first light source, hf≥φ, so photons supply enough energy to overcome the work function and electrons are emitted. When the frequency is lowered, hf<φ; each photon now has insufficient energy to free an electron, so no electrons are emitted even though the intensity (number of photons) is unchanged.
Examiner tips
- State that light is quantised into photons and give E=hf.
- Explain the role of the work function φ as the threshold energy.
- Show that hf≥φ for emission and hf<φ for no emission.
- Mention that intensity only affects the number of photons, not their energy.
Common mistakes
- Confusing intensity with photon energy.
- Forgetting to mention the work function as the threshold.
- Saying electrons are emitted at lower frequency without justification.
Mark scheme (5 marks)
- Light is made up of photons / light energy is quantised into photons
- Each photon has an energy that depends on its frequency (E = hf, so higher frequency means higher photon energy)
- The work function is the minimum energy needed to release an electron from the metal surface
- With the first source, photon energy is greater than or equal to the work function, so electrons are released
- With the lower frequency source, each photon has insufficient energy to overcome the work function, so no electrons are emitted regardless of intensity
Key terms in this question
photon · work function · frequency · intensity
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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