Huygens' Wave Theory
While Newton was writing up his corpuscular theory of light, the Dutch physicist Christiaan Huygens was developing a completely different picture — light as a spreading wave. It explained the same everyday behaviour just as well, made one crucial prediction that flatly contradicted Newton's, and still lost the argument for over a hundred years.
Light as a spreading disturbance
Christiaan Huygens proposed, in his 1690 Treatise on Light, that light is not a stream of particles at all but a wave — a disturbance spreading outward through a medium, in much the same way ripples spread outward across the surface of a pond after a stone is dropped in. On this view, a luminous source doesn't fire out particles but instead sets up a continuous, spreading wavefront, and what the eye detects as light is this disturbance arriving and passing through it.
To make this picture precise enough to predict how light actually behaves when it meets an obstacle or a boundary, Huygens introduced what is now called Huygens' principle: every point on a wavefront can be treated as a source of its own new, small secondary wavelet, spreading outward from that point at the wave's normal speed. The overall new wavefront a short time later is found by combining (taking the envelope of) all of these secondary wavelets together. This construction — building up a wave's future shape from an enormous number of tiny secondary sources spread across its current shape — turned out to be a genuinely powerful and general tool, and it remains the standard way of explaining diffraction and constructing wavefronts in wave optics today.
Explaining reflection and refraction — just as well as Newton's theory
Using the wavelet construction, Huygens could reproduce both of the basic behaviours of light already known at the time, just as successfully as Newton's corpuscular theory.
- Reflection. A wavefront meeting a mirror generates secondary wavelets that combine to send the wave back at an angle equal to the angle it arrived at, correctly reproducing the law of reflection.
- Refraction. Huygens explained bending at a boundary between two media by proposing that light travels at different speeds in different media — in particular, that it travels more slowly in a denser medium such as glass or water than in air. As a wavefront crosses the boundary at an angle, the part of the wavefront that enters the denser medium first is slowed down while the rest is still travelling at the original (faster) speed, which bends the whole wavefront's direction of travel — correctly reproducing the observed bending of a light ray toward the normal on entering a denser medium.
Here is the crucial disagreement between the two theories. Newton's corpuscular theory required light to travel faster in a denser medium (the medium's attraction speeds the corpuscles up); Huygens' wave theory required light to travel slower in a denser medium (the wave is slowed as it enters). Both theories reproduced the same observed bending at the boundary using opposite underlying speed changes — so in principle, a direct measurement of the speed of light in, say, water compared to air, would settle which theory was correct. In practice, the technology of the seventeenth and eighteenth centuries could measure the speed of light at all only with great difficulty, let alone compare it precisely between two different media — so this genuinely decisive test could not actually be carried out by either side at the time.
Why the wave theory lost out anyway
Since Huygens' wave theory explained reflection and refraction with equal success, and made a testable prediction that simply couldn't be tested yet, the choice between the two theories for the next century came down to the same factors discussed in the previous lesson — only now working against the wave picture rather than for it.
Authority. Newton's towering reputation weighed against Huygens just as it weighed in favour of the corpuscular theory generally; a competing theory needed to overcome that reputation, not merely match it on the physics.
Sharp shadows. This was the wave theory's most serious apparent weakness. If light really is a wave, ordinary wave behaviour says it should diffract — bend and spread out — around the edges of obstacles and through gaps, in the same visibly obvious way that water waves bend around a harbour wall. Everyday observation showed nothing of the kind: shadows had crisp, straight edges, exactly as the corpuscular theory predicted directly and the wave theory seemed, on the face of it, to struggle with.
The subtlety of the effects. The wave theory's real answer to the shadow problem — that light's wavelength is simply too short for its diffraction to be visible without careful, close apparatus — was correct, but it wasn't something seventeenth-century experimenters were equipped to demonstrate convincingly. Without the fine slits and precise observation needed to reveal diffraction and interference clearly, the wave theory's strongest piece of supporting evidence remained effectively invisible, while its apparent weakness (no obviously bending shadows) remained in plain view every day.
Considered purely on the evidence available in the late seventeenth century, neither theory was decisively wrong — both explained reflection and refraction, both made a prediction about the speed of light in a denser medium that nobody could yet test, and both had to contend with the same unresolved puzzle of why light's wave-like spreading (if it had any) wasn't visible in everyday shadows. The dispute stayed open, tilted toward Newton's corpuscles by authority and apparent shadow evidence, for the best part of the next hundred years.
What finally began to shift the balance was a much more careful, deliberately designed experiment — one built specifically to reveal light's subtle wave behaviour rather than rely on everyday observation. That experiment, Young's double-slit experiment, is the subject of the next lesson.
Key takeaways
- Huygens proposed light is a wave — a spreading disturbance — rather than a stream of particles.
- Huygens' principle treats every point on a wavefront as a source of secondary wavelets, whose combined envelope gives the new wavefront a moment later.
- This construction explains reflection and refraction just as successfully as Newton's corpuscular theory.
- The wave theory requires light to travel slower in a denser medium — the exact opposite of the corpuscular theory's prediction of a faster speed.
- This disagreement was, in principle, a decisive test, but seventeenth- and eighteenth-century apparatus couldn't measure and compare the speed of light precisely enough to use it.
- The wave theory lost out for over a century for the same reasons the corpuscular theory won: Newton's authority, the apparent problem of sharp (non-diffracting) shadows, and the lack of apparatus sensitive enough to reveal light's genuine but subtle wave effects.
- Young's double-slit experiment, covered next, was the first careful experiment designed to reveal those subtle wave effects directly.