AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Photon model of electromagnetic radiation, the Planck constant.
Full specification wording
Particles, antiparticles and photons: Photon model of electromagnetic radiation, the Planck constant.
Electromagnetic radiation can be modelled as a stream of photons, each carrying a discrete quantum of energy.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Electromagnetic radiation can be modelled as a stream of photons, each carrying a discrete quantum of energy. The energy of one photon is directly proportional to the frequency of the radiation, with the constant of proportionality being the Planck constant h. This gives the relationship E = hf, where E is photon energy in joules, h is the Planck constant and f is frequency in hertz. Because c = fλ for electromagnetic waves in a vacuum, the photon energy can also be written as E = hc/λ.
For example: A photon of green light with frequency 6.0 × 10¹⁴ Hz has energy hf, which is about 4.0 × 10⁻¹⁹ J. The model explains the photoelectric effect and line spectra, where energy is exchanged in discrete amounts.
What earns the marks
- State that electromagnetic radiation can be modelled as photons, each photon being a discrete quantum of energy.
- Recall and apply the relationship E = hf, identifying E as photon energy, h as the Planck constant and f as frequency.
- Use the wave equation c = fλ to derive or apply the alternative form E = hc/λ for a photon in a vacuum.
- Carry out calculations involving photon energy, frequency, wavelength and the Planck constant with consistent SI units.
- Explain that the photon model accounts for discrete energy exchanges in phenomena such as the photoelectric effect and line spectra.
Common mistakes and how to put them right
Confusing the Planck constant h with Planck's constant divided by 2π.
Put it right by using h as stated in the specification relationship E = hf.
Using wavelength directly in E = hf without converting to frequency.
Put it right: First using c = fλ or by using E = hc/λ.
Treating photon energy as continuous rather than quantised.
Put it right by stating that each photon carries a discrete energy hf.
Exam tips
- Write down the equation E = hf and, if wavelength is given, convert to frequency using c = fλ before substituting values.
- Check that frequency is in hertz and wavelength is in metres before calculating photon energy in joules.
- Quote the Planck constant with its unit J s and keep powers of ten consistent throughout the calculation.
Try the lesson
The start of the Particles, antiparticles and photons: Photon model of electromagnetic radiation, the Planck constant. lesson, made from checked teaching notes for this specification point.
Have a go first
Which equation gives the energy of a photon in terms of its frequency?
Listen
Electromagnetic radiation can be modelled as a stream of photons, each carrying a discrete quantum of energy. The energy of one photon is directly proportional to the frequency of the radiation, with the constant of proportionality being the Planck constant h. This gives the relationship E = hf, where E is photon energy in joules, h is the Planck constant and f is frequency in hertz. Because c = fλ for electromagnetic waves in a vacuum, the photon energy can also be written as E = hc/λ. For example, a photon of green light with frequency 6.0 × 10¹⁴ Hz has energy hf, which is about 4.0 × 10⁻¹⁹ J. The model explains the photoelectric effect and line spectra, where energy is exchanged in discrete amounts. A strong answer earns marks for these points. State that electromagnetic radiation can be modelled as photons, each photon being a discrete quantum of energy. Recall and apply the relationship E = hf, identifying E as photon energy, h as the Planck constant and f as frequency. Use the wave equation c = fλ to derive or apply the alternative form E = hc/λ for a photon in a vacuum. Carry out calculations involving photon energy, frequency, wavelength and the Planck constant with consistent SI units. A common slip: Confusing the Planck constant h with Planck's constant divided by 2π. Instead, using h as stated in the specification relationship E = hf. Exam tip: Write down the equation E = hf and, if wavelength is given, convert to frequency using c = fλ before substituting values.
Key words
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Photon
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Planck constant
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Energy–frequency relationship
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Photon model
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See every definition
- Photon
- Electromagnetic radiation can be modelled as a stream of these, each carrying a discrete quantum of energy.
- Planck constant
- The constant of proportionality between the energy of one photon and the frequency of the radiation, denoted h.
- Energy–frequency relationship
- The energy of one photon is directly proportional to the frequency of the radiation, giving E = hf.
- Photon model
- A model explaining the photoelectric effect and line spectra, where energy is exchanged in discrete amounts.
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