A student shines ultraviolet light onto a zinc plate. The ultraviolet light causes electrons to be released from the zinc plate. The student then replaces the ultraviolet light with visible red light of the same intensity. No electrons are released. Explain why ultraviolet light releases electrons from the zinc plate but red light of the same intensity does not.
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, where h is Planck’s constant and f is the frequency.
Ultraviolet light has a higher frequency than red light, so each ultraviolet photon carries more energy than each red photon.
A minimum amount of energy, the work function of zinc, is required to release an electron.
Each red photon does not have enough energy to reach the work function, and photons cannot combine their energies, so no electrons are released.
The energy of a photon is given by E=hf, where h is Planck’s constant and f is the frequency.
Ultraviolet light has a higher frequency than red light, so each ultraviolet photon carries more energy than each red photon.
A minimum amount of energy, the work function of zinc, is required to release an electron.
Each red photon does not have enough energy to reach the work function, and photons cannot combine their energies, so no electrons are released.
Examiner tips
- Use the equation E=hf to link frequency and photon energy.
- Explain the concept of a work function and why it matters.
- Mention that photons cannot combine energies in the photoelectric effect.
Common mistakes
- Confusing wavelength with energy; remember energy ∝ frequency, not wavelength. Assuming intensity matters; intensity only affects number of photons, not energy per photon. Saying photons can combine energies; in the photoelectric effect each electron requires a single photon of sufficient energy.
Mark scheme (5 marks)
- Light travels as discrete packets of energy called photons
- The energy of a photon depends on its frequency
- Ultraviolet light has a higher frequency than red light, so each ultraviolet photon carries more energy than each red photon
- A minimum amount of energy is needed to release an electron from the zinc plate
- Each red photon does not have enough energy to release an electron, and photons cannot combine their energies, so no electrons are released
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