Quantum physics
Some interactions transfer energy in discrete amounts. Photon energy links frequency to the photoelectric effect, while diffraction reveals the wave behaviour of particles.
Specification outcomes (16)
- 4.5.1(a) Describe the particulate (photon) nature of electromagnetic radiation.
- 4.5.1(b) Define a photon as a quantum of energy of electromagnetic radiation.
- 4.5.1(c) Use E = hf and E = hc / λ to calculate energy of a photon.
- 4.5.1(d) Define the electronvolt (eV) as a unit of energy.
- 4.5.1(e)(i) Use LEDs and eV = hc / λ to estimate the value of Planck’s constant.
- 4.5.1(e)(ii) Determine Planck’s constant using different coloured LEDs experimentally.
- 4.5.2(a)(i) Describe and explain the photoelectric effect and its experimental demonstration.
- 4.5.2(a)(ii) Demonstrate the photoelectric effect using apparatus such as a gold-leaf electroscope and zinc plate.
- 4.5.2(b) Explain that photoelectric emission involves a one-to-one interaction between a photon and a surface electron.
- 4.5.2(c) Use Einstein’s photoelectric equation hf = Φ + KEmax.
- 4.5.2(d) Define work function and threshold frequency.
- 4.5.2(e) State that maximum kinetic energy of photoelectrons is independent of radiation intensity.
- 4.5.2(f) State that rate of emission of photoelectrons above threshold frequency is proportional to radiation intensity.
- 4.5.3(a) Describe and explain electron diffraction as evidence of wave-like behaviour of particles.
- 4.5.3(b) Describe diffraction of electrons through thin polycrystalline graphite and relate to atomic spacing.
- 4.5.3(c) Use the de Broglie equation λ = h / p to relate momentum and wavelength.
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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