Wave OpticsIntroduction
Riya drops a pencil into a glass of water, and it seems to snap at the surface. You know the reason from the last chapter: light bends as it crosses from water into air, and Snell's law tells you by how much. But what is light, that it bends like this?
In 1637 Descartes pictured light as a stream of tiny particles and used the idea to derive Snell's law. Newton developed the picture in his famous book Opticks, so the corpuscular (particle) model is usually credited to him. In 1678 the Dutch physicist Christiaan Huygens proposed instead that light is a wave. Both models explain reflection and refraction, but they disagree about something that can be measured: how fast light travels in water, where a ray bends towards the normal. Ask each model, then run the test.
The particle model needs light to speed up when it bends towards the normal; the wave model needs it to slow down. Léon Foucault measured the speed of light in water in 1850 and found it less than in air, just as the wave model predicts. Even so, the wave theory was slow to win acceptance, partly because of Newton's authority and partly because a wave seemed to need a medium to travel through, while light crosses the vacuum of space.
From Particles to Waves
The wave model took more than two centuries to win. Put these steps in order, earliest first.
Young's experiment of 1801 firmly established that light is a wave, and for the next forty years or so one interference and diffraction experiment after another confirmed it. The last puzzle, how a wave can cross empty space, was solved by Maxwell: light is an electromagnetic wave, a pair of electric and magnetic fields that change in time and keep each other going, with no material medium at all (Chapter 8). Hertz produced such waves in his laboratory around 1890, and J. C. Bose and G. Marconi soon put them to practical use.
Why Light Seems to Travel in Rays
If light is a wave, why does it cast sharp shadows and travel in straight lines, while sound goes round corners so easily? Because its wavelength is tiny. Yellow light has a wavelength of about 0.6 μm, that is 6 × 10⁻⁷ m. Across a window just 1 mm wide, about
Mirrors, lenses and doorways are thousands to millions of wavelengths across, so the wave nature of light hardly shows and light seems to travel in straight lines. Geometrical optics, the ray optics of Chapter 9, is the limit in which the wavelength is taken to be zero, and a ray is the path along which energy travels in that limit. In this chapter we put the wavelength back and see what it does:
first Huygens' construction, and how it gives the laws of reflection and refraction (Sections 10.2 and 10.3); then interference, which rests on the principle of superposition (Sections 10.4 and 10.5); diffraction, explained by the Huygens–Fresnel principle (Section 10.6); and finally polarisation, which shows that light waves are transverse (Section 10.7).