9478 / 2027

Chapter 19

Quantum Physics

Separate what is detected from the model used to predict it, and connect each equation to the quantity it describes.

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  1. Evidence for photons

    Explain threshold and wave evidence, then calculate photon energy, momentum and the meaning of intensity.

  2. Matter waves and detections

    Distinguish electron diffraction from accumulated single-particle interference and apply the de Broglie relation.

  3. Wavefunctions and probability

    Distinguish signed amplitude, density and interval probability, and normalise square and sinusoidal profiles.

  4. Adding probability amplitudes

    Add coherent amplitudes before squaring, interpret relative signs and connect superposition to allowed standing shapes.

  5. Localisation and momentum spread

    Explain why a localised state needs a spread of momenta and estimate the corresponding width scales.

  6. A particle in a box

    Use infinite-well boundary conditions to identify standing wavefunctions, nodes and allowed energies.

  7. Atomic transitions and spectra

    Connect discrete electronic levels and occupied initial states to emission, absorption and photon energy gaps.

Syllabus coverage

This chapter covers Quantum Physics, topic 19(a-m), in the 9478 syllabus (2027).

Choose an appropriate quantum model, calculate particle and photon quantities, interpret probability graphs and explain discrete energy changes.

Photons and matter waves, normalised wavefunctions, amplitude superposition, localisation, the infinite square well and atomic line spectra.

View the learning outcomes
19(a)
Explain how photoelectric threshold evidence supports particulate radiation while interference and diffraction support its wave nature. Evidence for photons
19(b)
Define a photon as a quantum of electromagnetic radiation and use E = hf. Evidence for photons
19(c)
Explain that a massless photon has momentum and use p = E/c = h/lambda. Evidence for photons
19(d)
Explain the wave evidence from electron diffraction and double-slit interference of single particles. Matter waves and detections
19(e)
Use lambda = h/p to solve de Broglie wavelength problems. Matter waves and detections
19(f)
Represent a particle state by a wavefunction, interpret amplitude squared as probability density, including an atomic electron cloud, and normalise square and sinusoidal profiles. Wavefunctions and probability
19(g)
Apply superposition to position wavefunctions to explain single-particle interference and standing solutions in a box. Adding probability amplitudesA particle in a box
19(h)
Explain localisation through a necessary momentum spread and apply the position-momentum width relation. Localisation and momentum spread
19(i)
Explain standing wavefunctions for a particle in a one-dimensional infinite square well. A particle in a box
19(j)
Use E_n = h^2 n^2/(8mL^2) for allowed infinite-well energies. A particle in a box
19(k)
Explain discrete electronic levels in isolated atoms, including hydrogen, and their connection to spectral lines. Atomic transitions and spectra
19(l)
Distinguish emission and absorption line spectra. Atomic transitions and spectra
19(m)
Calculate photons absorbed or emitted during atomic energy transitions. Atomic transitions and spectra