A student is investigating the emission of light from a LED (light-emitting diode). The student knows that electrons inside the LED lose energy and that this energy is released as photons. The LED emits green light of frequency 5.5 × 10¹⁴ Hz. The student is told that a second LED emits photons of lower frequency than the green LED. Explain what this tells us about the energy of each photon emitted by the second LED, and describe how the energy of a photon is related to its frequency.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A LED converts electrical energy into light by releasing photons when electrons lose energy inside the device.
Model answer (5 marks)
A photon is a packet of electromagnetic energy.
The energy of a photon is directly proportional to its frequency: E = hf, where h is Planck’s constant.
Because the second LED emits photons of lower frequency than the green LED, each photon it emits has lower energy.
Thus each electron in the second LED loses less energy when it releases a photon than an electron in the green LED.
This lower energy loss corresponds to the lower frequency of the emitted light.
The energy of a photon is directly proportional to its frequency: E = hf, where h is Planck’s constant.
Because the second LED emits photons of lower frequency than the green LED, each photon it emits has lower energy.
Thus each electron in the second LED loses less energy when it releases a photon than an electron in the green LED.
This lower energy loss corresponds to the lower frequency of the emitted light.
Examiner tips
- Use the equation E = hf to link energy and frequency.
- State that lower frequency means lower energy.
- Mention that the electron loses less energy in the second LED.
- Show the proportionality clearly.
Common mistakes
- Confusing wavelength with frequency.
- Using E = hc/λ instead of E = hf.
- Failing to explain that lower frequency gives lower energy.
Mark scheme (5 marks)
- A photon is a packet (quantum) of electromagnetic energy
- The energy of a photon is directly proportional to its frequency (E ∝ f / E = hf)
- h is the Planck constant (a fixed/constant value)
- The second LED emits photons of lower energy than the green LED
- Each electron in the second LED loses less energy when it releases a photon (compared to the green LED)
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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