Models of light
Particles (Newton) → waves in an aether (Huygens, Young, Fresnel) → electromagnetic waves (Maxwell, Hertz) → photons (Planck, Einstein). Today: wave–particle duality. Light propagates as a wave and exchanges energy as particles.
Which model explains what?
Tap a phenomenon row to see the deciding experiment.
| Phenomenon | Newton particles | Huygens waves | Maxwell EM | Photons |
|---|
Tap a row.
Labs
Move the sliders and watch the physics respond. Each lab matches an exam calculation.
Fringe spacing Δy = λL/d
Malus' law I = I₁ cos²θ
Wien's law and the ultraviolet catastrophe
Ek(max) = hf − φ
Try it: pick zinc with visible light, then turn the intensity to maximum. Nothing happens. A wave model says brighter light should eventually free electrons; photons say each one must carry at least φ.
Measuring the speed of light
Every method uses c = d/t or c = fλ. History moved from astronomical distances (Rømer) to kilometres with spinning wheels and mirrors, then to centimetres with microwaves and lasers. Since 1983, c = 299 792 458 m/s exactly, by definition.
Each new method cut the uncertainty by orders of magnitude
Relative uncertainty (log scale). Hover or tap a point for the value.
1983: c fixed exactly, so uncertainty is zero by definition (not plotted).
Fizeau and Michelson
Why modern methods are more accurate
- Time measured electronically against atomic clocks (about 1 part in 10¹⁵), not by eye or by spinning machinery.
- Microwaves and lasers are monochromatic and coherent, so λ is found precisely by interference.
- Short, controlled paths in a vacuum or lab remove air-density, weather and surveying errors.
- No human judgement of when an image disappears or stops moving.
Exam structure for any method: what was measured → formula → result → main uncertainty → how the next method improved.
Models of the atom
Solid ball (Dalton) → plum pudding (Thomson) → nucleus with orbiting electrons (Rutherford) → quantised energy levels (Bohr) → probability cloud (Schrödinger), with the neutron completing the nucleus (Chadwick). Every change was forced by an experiment.
Pick a jump, get the photon
1/λ = R(1/n_f² − 1/nᵢ²)
Model summary
| Model | Explains | Could not explain | Overturned by |
|---|---|---|---|
| Dalton 1803 | Chemical laws, fixed ratios | Electrons, ions, isotopes | Thomson's cathode rays (1897) |
| Thomson 1904 | Electrons in all atoms; neutral atoms | Large-angle scattering; spectra | Geiger–Marsden (1909) |
| Rutherford 1911 | Scattering; tiny dense nucleus | Stability; line spectra; nuclear mass | Bohr (1913); Chadwick (1932) |
| Bohr 1913 | Stability (by postulate); hydrogen spectrum | Multi-electron atoms; line intensities; Zeeman effect | de Broglie, Schrödinger (1924–26) |
| Schrödinger 1926 | Multi-electron atoms; periodic table; probabilities | Relativity; nuclear structure | Still used (refined by quantum field theory) |
The Standard Model of matter
All ordinary matter is built from 12 fundamental fermions (6 quarks, 6 leptons) that interact by exchanging force-carrier bosons. The Higgs boson gives particles mass. Three forces are included (strong, electromagnetic, weak); gravity is not.
Tap a particle
Columns I, II, III are generations: same charge, increasing mass. Ordinary matter uses only generation I (u, d, e⁻, νₑ).
| Force | Boson | Mass | Range | Relative strength | Acts on |
|---|---|---|---|---|---|
| Strong | Gluon (8 types) | 0 | ~10⁻¹⁵ m | 1 | Colour charge: quarks, gluons |
| Electromagnetic | Photon γ | 0 | Infinite | ~10⁻² | Electric charge |
| Weak | W⁺, W⁻, Z⁰ | 80–91 GeV/c² | ~10⁻¹⁸ m | ~10⁻⁶ | All fermions; changes flavour |
| Gravity (not in SM) | Graviton (hypothetical) | 0 | Infinite | ~10⁻³⁸ | Mass–energy |
Combine quarks: baryons (qqq) or mesons (qq̄)
β⁻ decay at the quark level
| Conserved quantity | n | p + e⁻ + ν̄ₑ | |
|---|---|---|---|
| Charge | 0 | +1 − 1 + 0 = 0 | ✓ |
| Baryon number | +1 | +1 + 0 + 0 = +1 | ✓ |
| Electron lepton number | 0 | 0 + 1 − 1 = 0 | ✓ |
Only the weak force changes quark flavour. Pauli predicted the antineutrino (1930) because beta electrons had a spread of energies.
Evidence timeline
Strengths
- Explains all ordinary matter with 17 particles.
- Predicted W, Z, charm, top and Higgs before discovery.
- Matches experiment to extraordinary precision (electron magnetic moment, ~1 part in 10¹²).
- Explains hadrons as quark combinations and the "particle zoo".
Limitations
- No gravity.
- No dark matter (~27%) or dark energy (~68%).
- Assumed massless neutrinos; oscillations (1998) show they have mass.
- Cannot explain matter–antimatter imbalance.
- About 19 free parameters measured, not predicted.
The same method repeats from Rutherford to SLAC: fire high-energy particles at a target, study how they scatter, infer the structure. 5 MeV alphas found the nucleus; 20 GeV electrons found quarks.
Exam practice
Calculations, short answers, and full 6–9 mark extended responses with marking criteria and band 6 model answers. Try each before opening the solution.
h = 6.626×10⁻³⁴ J s · c = 3.00×10⁸ m/s · e = 1.602×10⁻¹⁹ C · mₑ = 9.109×10⁻³¹ kg · b = 2.898×10⁻³ m K · R = 1.097×10⁷ m⁻¹
How to score band 6 on long answers
- Plan for two minutes: scientist, date, experiment, result, link to model.
- Open with a judgement when the verb is assess or evaluate.
- One paragraph per model: context → experiment → observation → what it showed → limitation → next model.
- Use equations and a quick labelled sketch where they prove a point.
- Close with a judgement that answers the question directly.