A-Level Physics: Quantum Phenomena

Quantum phenomena is where A-Level Physics first breaks with classical intuition, and exam questions test both the calculations and the reasoning about why the classical picture fails. This quiz covers the full examinable core: photoelectric effect, energy levels, and wave-particle duality.

The photoelectric questions work the Einstein equation with real numbers: maximum kinetic energy from photon energy minus work function (a calculation whose answer key was independently recomputed and corrected during verification — 0.77 × 10⁻¹⁹ J), the work function from threshold frequency, and the factors that genuinely set the stopping potential. The conceptual heart of the topic gets its own multi-select question: exactly which experimental facts — instant emission, the existence of a threshold frequency — defeat the classical wave account of light.

Energy-level questions handle both directions of the photon bookkeeping: the frequency of the photon emitted in a hydrogen n=3 to n=1 transition (converting electronvolts to joules before dividing by h), and the minimum photon energy needed for an upward transition between given levels. A discrete-spectra question consolidates why line spectra exist at all.

The duality questions close the set: de Broglie wavelength from momentum directly, and from an accelerating potential difference via the momentum-energy chain; and a scenario-selection question — expanded during verification — recognising electron diffraction, electron interference and the photoelectric effect as the three canonical demonstrations of wave-particle duality, against decoys that demonstrate nothing quantum at all.

Every value needed is given in the question, matching the data-booklet reality of the exam, and each explanation shows the complete unit-consistent calculation.

• Apply Einstein's photoelectric equation to find maximum kinetic energy and work function
• Explain which photoelectric observations defeat classical wave theory
• Convert between eV and joules and compute photon frequencies for level transitions
• Calculate de Broglie wavelengths from momentum or accelerating potential
• Identify the canonical experimental evidence for wave-particle duality

Topic scope follows section 3.2.2 (Electromagnetic radiation and quantum phenomena) of the current A-level physics subject content: the photoelectric effect, ionisation and excitation, energy levels and line spectra, and wave-particle duality.