Light: Waves or Particles?
A three-hundred-year argument, and what light turned out to be.
Lesson at a glanceShow the key points
- Newton: light is a stream of particles. Huygens: light is a wave. Young's double slit (1801), Fresnel's diffraction experiments, Foucault's speed of light in water and Maxwell's theory settled it as a wave; the photoelectric effect later showed a particle side too (wave–particle duality).
- Light is a non-mechanical wave: it needs no medium (no 'ether'). The quantity that oscillates is the electric field, transverse to the direction of travel, so light can be polarized.
- Plane wave E = E₀ sin ω(t − x/v) (17.1). Speed in vacuum c ≈ 3 × 10⁸ m/s for every observer.
- Refractive index μ = c/v (17.2). A point source gives E ∝ 1/r, so intensity ∝ 1/r².
- Visible light: about 380 nm (violet) to 780 nm (red). Monochromatic light has a single (in practice dominant) wavelength; white light is a mixture.
- Since λ is tiny compared with everyday openings, diffraction is negligible and light can be treated as rays: rectilinear propagation, reflection (i = r) and refraction (sin i / sin r = v₁/v₂).
Is light a stream of tiny particles or a wave? The question kept physicists arguing for three centuries, and the answer turned out to be stranger than either side expected.
A long argument
Newton pictured light as a stream of tiny corpuscles fired out by a source and travelling in straight lines. Sharp shadows seemed to support this. Reflection became corpuscles bouncing elastically off a surface; refraction became corpuscles being pulled by the particles of glass or water as they entered.
Huygens, working at the same time, proposed that light is a wave. Its straight-line travel would then just mean that its wavelength is far smaller than ordinary openings and obstacles. Newton’s reputation kept the particle picture on top for about a century, until a run of results settled the matter:
- Young’s double-slit experiment (1801) showed light from two coherent sources producing bright and dark bands: interference.
- Fresnel’s experiments explained diffraction in detail.
- Foucault (1850) found light travels slower in water than in air, as the wave theory predicts and the corpuscle theory doesn’t.
- Maxwell’s theory of electromagnetic waves (1860s) predicted the speed of light correctly.
Then came a twist. Around 1900 it was found that light falling on a metal knocks out electrons whose energy doesn’t depend on how bright the light is: the photoelectric effect. Einstein explained it in 1905 only by treating light as particles. Today we accept that light has a dual character: it behaves as a wave in some situations and as particles in others. (Even electrons turn out to show interference.) This chapter is about the wave side.
What is waving?
A wave on a string moves the string; a sound wave changes the pressure of air. Light is different: it needs no medium and crosses the empty space between the sun and the earth. Early physicists, unable to imagine a wave without a medium, invented an all-filling elastic “ether”. It was never found.
What oscillates in a light wave is the electric field, a vector quantity you’ll meet properly in later chapters. It points across the direction of travel, so light is a transverse wave and can be polarized. For a plane wave moving along x with the field along a fixed direction:
The speed of light in vacuum, , is the same for every observer, however they move. (This surprising fact is the starting point of the special theory of relativity.) In a transparent material light is slower:
For a point source the waves spread as spheres, : the amplitude falls as 1/r, so the intensity falls as 1/r².
A flint glass has refractive index 1.6. How fast does light travel in it?
Colours and the visible spectrum
The eye responds to frequencies from about 3.8 × 10¹⁴ Hz to 7.8 × 10¹⁴ Hz, which is wavelengths from about 780 nm (red) down to 380 nm (violet). The colour we sense is linked to the wavelength:
| Colour | Wavelength (roughly) |
|---|---|
| Red | 620–780 nm |
| Orange | 590–620 nm |
| Yellow | 570–590 nm |
| Green | 500–570 nm |
| Blue | 450–500 nm |
| Violet | 380–450 nm |
Monochromatic and white light
Light of a single wavelength is monochromatic. No real source is perfectly monochromatic (even a laser has a tiny spread), so we use the word for light with one dominant wavelength. Most sources emit a mixture, and the eye blends the mixture into one of a huge range of colours. White light is a mixture of all visible wavelengths in suitable proportions.
When can we use rays?
Waves bend round obstacles and spread from openings, strongly when these are comparable to the wavelength. Light’s wavelength is thousands of times smaller than everyday objects, so for most purposes diffraction is negligible. Light then travels in straight rays and casts sharp shadows. This geometrical optics approximation rests on three rules:
- Rectilinear propagation: light travels in straight lines in a uniform medium.
- Reflection: angle of incidence = angle of reflection, and the incident ray, reflected ray and normal are coplanar.
- Refraction: , with the incident ray, refracted ray and normal coplanar.
- 1Newton: particles; Huygens: waves. Interference, diffraction, slower light in water and Maxwell's theory established waves; the photoelectric effect revealed a particle side too.
- 2Light needs no medium; its electric field oscillates transverse to the direction of travel.
- 3E = E₀ sin ω(t − x/v); c ≈ 3 × 10⁸ m/s for every observer; μ = c/v.
- 4Visible light: about 380 nm (violet) to 780 nm (red); white light is a mixture.
- 5When λ is tiny compared with obstacles, light travels as rays obeying the laws of reflection and refraction.
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