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The wave-particle debate over the nature of light is the longest-running unresolved controversy in physics. It began in the 1670s, ran for almost 150 years before a decisive experiment chose one side, and then — through a strange and beautiful twist — was reopened in 1905 by Einstein and resolved only by the modern wave-particle duality of quantum mechanics. This lesson is about the first half of that story: Newton's corpuscular theory of light as set out in Opticks (1704), Huygens' wave-front construction (1678 manuscript, 1690 Traité de la lumière), the two theories' incompatible predictions for what happens to light entering a denser medium, and the 19th-century experimental results — Young's double slit and finally Foucault's measurement of the speed of light in water — that broke the stalemate in favour of waves. The point of including this in a physics syllabus is not history-for-its-own-sake. The two theories make opposite predictions about a single measurable quantity, the speed of light in glass relative to vacuum, and the experimental test is a model of how physics resolves competing models.
Spec mapping: This lesson covers part of AQA 7408 section 3.12.2 — wave-particle nature of light, the corpuscular theory (Newton), the wave theory (Huygens), each theory's prediction for the speed of light in a denser medium, and the 19th-century experimental evidence (Young 1801, Fresnel, Foucault 1850) that supported the wave theory. (Refer to the official AQA specification document for exact wording.)
Synoptic links:
- Section 3.3 (waves): Huygens' principle — every point on a wavefront acts as a source of secondary wavelets — is the foundation of A-Level diffraction and refraction. The familiar Year-12 derivation of Snell's law from secondary wavefronts is a direct application of Huygens.
- Section 3.12.2 (Maxwell, photoelectric effect): the wave theory's 19th-century triumph (Young, Fresnel, Foucault, Maxwell) sets the stage for the re-introduction of a particle picture by Einstein in 1905 — the photoelectric effect (lesson 4) is meaningful only against the backdrop of a century of wave-theory success.
- Section 3.2 (quantum phenomena): the modern wave-particle duality of photons resolves the original Newton-Huygens dispute by saying that both sides were partly right — light is neither a classical wave nor a classical particle, but a quantum entity displaying either behaviour depending on the apparatus.
Isaac Newton's Opticks (1704) proposed that light consists of tiny particles — "corpuscles" — emitted by luminous bodies and travelling in straight lines through space and through transparent media. The particles were imagined as having mass (or at least inertia) and possibly different sizes/colours corresponding to the colours of the spectrum. The theory was attractive for several reasons:
The interesting and (ultimately) lethal prediction concerned refraction at the boundary into a denser medium. Newton imagined the corpuscles being attracted, perpendicularly to the surface, by the denser medium — an attractive force that pulled them in. The transverse component of velocity (parallel to the surface) would be unchanged, but the normal component would be increased. The net result: the particles travel faster in the denser medium than in vacuum or air. Snell's law n = sin i / sin r emerges from this with the identification
Newton: v_glass / v_air = n, so n = 1.5 implies v_glass = 1.5 × v_air.
This was not a side prediction — it was central. A particle theory that pulls corpuscles into glass requires them to speed up.
Christiaan Huygens' Traité de la lumière (written 1678, published 1690) modelled light as a disturbance propagating through a luminiferous medium — the aether — in which every point on a wavefront acts as a source of secondary spherical wavelets whose envelope is the next wavefront. This Huygens' principle is now a standard tool in A-Level physics:
Every point on a wavefront acts as a point source of secondary spherical wavelets, propagating into the medium ahead of the wavefront with the local wave speed; the envelope of these secondary wavelets is the new position of the wavefront a short time later.
From this single rule, both reflection and refraction follow. For refraction at the boundary into a denser medium, the wavefront enters the new medium with one edge first; the part already in the denser medium propagates at the slower speed v₂ < v₁, while the part still in the original medium propagates at v₁. The result is that the wavefront's leading edge "pivots" toward the normal — i.e. the light bends toward the normal as it enters a denser medium, which is exactly what experiments show. Crucially, this construction yields:
Huygens: v_glass / v_air = 1/n, so n = 1.5 implies v_glass = v_air / 1.5.
In other words, Huygens predicts light goes slower in a denser medium. This is the opposite of Newton's prediction. Both theories reproduce Snell's law and the everyday observation that light bends toward the normal — but they make incompatible predictions about an underlying quantity that nobody, in the 1700s, could measure directly. Light was simply too fast.
| Phenomenon | Newton (corpuscle) | Huygens (wave) |
|---|---|---|
| Rectilinear propagation | Particles travel in straight lines | Plane wavefronts advance perpendicular to themselves |
| Reflection | Elastic collision with surface | Wavefront reverses normal component |
| Refraction toward normal | Attractive normal force in denser medium | Slower wave speed in denser medium |
| Speed in glass vs air | v_glass > v_air (n = v_glass/v_air) | v_glass < v_air (n = v_air/v_glass) |
| Interference / diffraction | No clean explanation | Natural — secondary wavelets superpose |
| Polarisation | Inherent particle asymmetry | Transverse wave (post-1817 Fresnel) |
| Newton's rings | Periodic "fits of easy reflection/transmission" — ad hoc | Path-difference interference between two surfaces |
Newton himself was aware of phenomena that fitted poorly into the corpuscular theory. Newton's rings — the coloured concentric fringes produced when a slightly curved glass lens rests on a flat plate — require some periodic property of the light, which the corpuscular theory could only handle by attributing alternating "fits of easy reflection" and "fits of easy transmission" to the corpuscles. This was widely seen as ad hoc even at the time. Newton's enormous personal authority kept the corpuscular theory dominant in Britain through the 18th century, but on the Continent — where Huygens and Euler were the better-known names — the wave theory had stronger support.
Thomas Young's double-slit experiment, presented to the Royal Society in 1801 and published in 1804, demonstrated interference fringes for visible light. Monochromatic light passing through two narrow, closely-spaced slits produces, on a distant screen, a pattern of alternating bright and dark bands whose spacing satisfies the standard fringe equation:
w = λD/s
where w is fringe spacing, λ is wavelength, D is the distance from slit to screen, and s is the slit separation. (Students meet this in section 3.3 of the spec.)
Interference requires the superposition of two coherent disturbances. Two streams of independent particles cannot cancel each other to produce darkness — but two waves of equal amplitude meeting out of phase can. Young's result was therefore very strong evidence for a wave theory of light. Newton's defenders attempted various particle-based explanations but none was convincing.
The reception was mixed. Young was attacked sharply by Henry Brougham in the Edinburgh Review — an anonymous demolition that delayed wide acceptance of the wave theory in Britain for two decades. It was the work of Augustin-Jean Fresnel in the 1810s and 1820s that finally tipped the balance, both because Fresnel reproduced Young's results with much more refined apparatus and because Fresnel's quantitative diffraction theory — including the famous "Poisson's spot" prediction (a bright point at the centre of the shadow of a circular disc, confirmed experimentally by Arago) — was so detailed and so accurate that the wave theory became the working consensus on the Continent in the 1820s and in Britain in the 1830s-40s.
graph LR
A["Monochromatic source"] --> B["Single slit<br/>(coherence)"]
B --> C["Double slits S₁, S₂"]
C --> D["Screen<br/>(fringes w = λD/s)"]
style C fill:#1d4ed8,color:#fff
style D fill:#27ae60,color:#fff
Even after Fresnel, the speed-of-light prediction had not been directly tested. The two theories were in dispute on a falsifiable, quantitative point — and the apparatus needed (a way to measure the speed of light in water and in air on a laboratory bench) became available only in the mid-19th century. Léon Foucault in 1850 used a rotating-mirror method to measure the speed of light in air and in water under the same conditions. (Hippolyte Fizeau, with whom Foucault initially collaborated, used a slightly different toothed-wheel method around the same time.)
The result was decisive: light travels slower in water than in air, by approximately the factor predicted by Huygens (water has refractive index n ≈ 1.33, and Foucault's measured speed ratio matched 1/1.33 to within experimental accuracy). Newton's corpuscular theory required the opposite — speed up by factor n in water — and was simply wrong.
By 1850, the wave theory of light was the experimentally established theory. Light was a wave. The remaining question — what kind of wave, in what medium — would be answered by Maxwell in 1865 (next lesson). The corpuscular theory was, for the time being, dead.
Light of wavelength 550 nm passes through a pair of slits 0.50 mm apart, and an interference pattern is observed on a screen 2.0 m away. Calculate the fringe spacing.
Solution. Using w = λD/s:
w = (550 × 10⁻⁹) × 2.0 / (0.50 × 10⁻³) = (1.10 × 10⁻⁶) / (5.0 × 10⁻⁴) = 2.2 × 10⁻³ m = 2.2 mm.
This is well within the resolution of the naked eye and was therefore observable with the simple apparatus available to Young in 1801. The result is meaningful only on the wave theory.
A flint glass has refractive index n = 1.62 at the sodium D-line wavelength (λ = 589 nm in vacuum). Calculate (a) the speed of light in this glass, and (b) the wavelength of the light inside the glass.
Solution.
(a) On the wave theory, n = c/v_glass, so v_glass = c/n = (3.00 × 10⁸) / 1.62 = 1.85 × 10⁸ m s⁻¹.
(b) The frequency is unchanged at the boundary; only the speed and wavelength change. λ_glass = v_glass / f. The frequency is f = c/λ_vac = (3.00 × 10⁸)/(589 × 10⁻⁹) = 5.09 × 10¹⁴ Hz. Wavelength in glass: λ_glass = (1.85 × 10⁸)/(5.09 × 10¹⁴) = 3.64 × 10⁻⁷ m ≈ 364 nm.
Equivalently, λ_glass = λ_vac / n = 589 / 1.62 = 364 nm.
On Newton's corpuscular theory, the predicted speed in glass would have been v_glass = c × n = 4.86 × 10⁸ m s⁻¹, which exceeds the speed of light in vacuum and is now known to be impossible.
It is tempting to ask why the Newton-Huygens debate was not settled in, say, 1750. The answer is a clean illustration of how physics depends on instrumentation. The decisive measurement — speed of light in water — required:
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