How We Came to Know the Cosmos: Light & Matter

# Chapter 5. 19th Century Wave Theories

## 5.1 Young’s double-slit experiments

The British natural philosopher Thomas Young provided strong evidence for Christiaan Huygens’ wave theory of light (discussed in Chapter 3) in 1803 when he published the results of his double-slit experiments.[1]

Young repeated earlier experiments with diffraction (discussed in Chapter 2) but passed the light through more than one slit. Young stated,

• If light is composed of particles, then they should each pass through a single slit, eventually creating two bright patterns on the other side.
• If light is composed of waves, then the slits will cause diffraction, and the light should produce a predictable interference pattern, just like water waves do.
 Figure 5.1Image credit The double-slit experiment with particles.
 Figure 5.2Image credit The double-slit experiment with waves.

Young showed that light behaves like a wave and creates an interference pattern, which is a consequence of the superposition principle.

 Figure 5.3Image credit Young’s sketch showing the results of his double-slit experiment, 1803.

### 5.1.1 The superposition principle

The superposition principle shows that when two waves meet, a new wave is created that has an amplitude equal to the sum of the amplitudes of the two waves it is composed of. This means that if two waves are emitted in the same phase, then they create a wave that is twice their former amplitude, and waves that are out of phase by 180° will become flat.

 Figure 5.4Image credit The superposition of waves in (left) and out (right) of phase.

Once Young knew that light is made of waves, he was able to estimate the wavelengths of individual colours using data from Isaac Newton.

## 5.2 Polarisation

In 1816, the French engineer Augustin-Jean Fresnel proposed that light waves have a transverse as well as longitudinal component, and Young made the same discovery independently, the following year.[2] Young and Fresnel went on to explain Newton’s results in terms of their wave theory and, by 1821, they were able to show that light waves are entirely transverse. This allowed them to explain the behaviour Huygens had documented in calcite crystals[3] (discussed in Chapter 3).

Calcite crystals act as polarisers, a term coined by the French mathematician Étienne-Louis Malus in about 1811, and this means that they only allow the ray to propagate in one plane, either horizontally or vertically.[4]

The horizontal component of a group of waves cannot travel through a vertically aligned polariser, and the vertical component cannot travel through a horizontally aligned polariser. When two crystals are placed next to each other, the light will only be able to travel through both if they are aligned the same way.

 Figure 5.5Image credit An electromagnetic wave.
 Figure 5.6Image credit Vertical polarisation.

People were able to study electricity in the laboratory for the first time in the early 19th century. The Italian natural philosopher Alessandro Volta created the first electric cell in 1800 by placing paper that had been soaked in salt-water between pieces of zinc and copper. This invoked a voltage, a difference in electrical energy between two points. By connecting many cells together, Volta was able to create a battery, known as a voltaic pile.[5]

 Figure 5.7Image credit A diagram of a voltaic pile.
 Figure 5.8Image credit A voltaic pile.

In 1831, the British natural philosopher Michael Faraday discovered that if a magnet is moved across a copper wire, then this also creates a current. This is known as Faraday’s law of induction and it was soon put to use in the invention of the electric motor.[6]

Faraday discovered the Faraday effect in 1845. This shows that a magnetic field can cause a ray of polarised light to rotate, a horizontal ray will become vertical and a vertical ray will become horizontal. This means that electricity, magnetism, and light must all be connected.[7]

In 1864, the British natural philosopher James Clerk Maxwell combined Faraday’s law of induction with three other equations:[8] the German mathematician Carl Friedrich Gauss’ two laws concerning electric and magnetic fields[9], and the French natural philosopher Andre-Marie Ampere’s law relating magnetic fields to electric current.[10]

Maxwell used these equations to develop an electromagnetic wave equation. The velocity of this wave was calculated to be the same as the speed of light, and so Maxwell concluded that light is a form of electromagnetic radiation.

Maxwell proposed that light is a transverse wave composed of oscillating electric and magnetic fields. Maxwell’s equations predicted that light could have an infinite number of wavelengths, suggesting that light must exist at energies well beyond the visible spectrum.

### 5.3.1 The electromagnetic spectrum

The British astronomer William Herschel had discovered infrared light in 1800. Herschel measured the temperature of different colours using prisms and found that the temperature was highest just beyond the colour red.[11] The German natural philosopher Johann Wilhelm Ritter predicted the existence of ultraviolet light in 1801 when he found that it reacts with silver chloride.[12]

The electromagnetic spectrum was completed by the end of the 19th century. The German physicist Heinrich Hertz demonstrated the existence of radio waves and microwaves by 1888,[13] the German physicist Wilhelm Röntgen discovered X-rays in 1895,[14] and the French chemist Paul Villard discovered gamma-rays in 1900.[15]

By 1900, the light bulb, the tram, and the telephone were invented and electrical transmission lines were developed that would soon allow the public to access electricity in their own homes.[16]

 Figure 5.9Image credit ‘First electric street lights in Berlin’, 1884 by Carl Saltzmann.
 Figure 5.10Image credit The electromagnetic spectrum, where blue light has a shorter wavelength than red light.

## 5.4 The aether

Maxwell’s electromagnetic theory, like all previous theories of light, relied on the idea that space is filled with a substance known as the aether. The aether was thought to be a medium, like air or water, that allows light waves to travel through space. As the Earth moves through space, it moves through the aether, and the light of the Sun can be dragged forwards in its direction of travel, like sound in the wind.

Galileo’s relativity (discussed in Book I) showed that speeds are additive. This means that if you walk at speed v across the deck of a ship that is moving at speed u, then someone standing on the shore will measure your speed to be v + u. When light from the Sun is travelling in the same direction as the aether, we should measure its speed to be c + a, where a is the velocity of the aether. When light travels in the opposite direction to the aether, we should measure its speed to be c - a.

 Figure 5.11Image credit The aether.

In 1887, the American physicists Albert Michelson and Edward Morley devised an experiment that was precise enough to measure the difference in the speed of light as the Earth moves around the Sun.[17] To almost everyone’s surprise, they found that the speed of light moves at the same rate in all directions. This meant that there was no aether, and so no explanation for how light can travel through space. This was explained in the 20th Century, with the discovery of quantum mechanics (discussed in Chapter 8).