Wave frequency and photon energy
Frequency describes a wave’s oscillations and determines the energy of each photon. For light in a vacuum, wavelength provides another route to photon energy.
Follow the reasoning
- Convert wavelength to metres.
- Use c = fλ to find frequency in a vacuum.
- Multiply frequency by Planck’s constant to find the energy per photon.
Specification outcomes (3)
- 4.4.1(b)(i) Define displacement, amplitude, wavelength, period, phase difference, frequency and speed of a wave.
- 4.4.1(d) Use the wave equation v = fλ.
- 4.5.1(c) Use E = hf and E = hc / λ to calculate energy of a photon.
H156 v2.0 · May 2026
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An optical sensor alternates between a green source of wavelength 520 nm and a red source of wavelength 650 nm. Treat each source as monochromatic and use its wavelength in a vacuum. Each source emits 2.0 μJ of light energy in a burst. Use c = 3.00 × 10⁸ m s⁻¹ and h = 6.63 × 10⁻³⁴ J s. (a) Calculate the frequency of the green light. [2] (b) Calculate the energy of one green photon and one red photon. [3] (c) Calculate the number of photons emitted in one green burst. [2] (d) A technician claims that equal-energy bursts contain equal numbers of photons. Calculate the ratio (red photon count)/(green photon count) and explain whether the claim is correct. [3]
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