British natural philosopher Thomas Young provided strong evidence for Dutch natural philosopher Christiaan Huygens' wave theory of light in 1803, when he published the results of his double-slit experiments.
Young repeated earlier experiments with diffraction but passed the light through more than one slit.
If light is composed of particles, then they should all pass through separate holes and create two bright patterns on the other side.
The double-slit experiment with particles. Image credit: inductiveload/Public domain.
The double-slit experiment with waves. Image credit: inductiveload/Public domain.
Young showed that light behaves like a wave and creates an interference pattern, which is a consequence of the superposition principle.
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 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.
Young's sketch of two-slit diffraction of light, 1803. Image credit: Thomas Young/Public domain.
Once Young knew that light is made of waves, he was able to estimate the wavelengths of individual colours using data from English natural philosopher Isaac Newton.
2. Polarisation ↑
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.
Calcite crystals act as polarisers, a term coined by 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.
The horizontal component of a wave 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.
3. Electromagnetic radiation ↑
People were able to study electricity in the laboratory for the first time in the early 1800s. Italian natural philosopher Alessandro Volta created the first electric cell in 1800. He achieved this 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.
A voltaic pile. Image credit: MdeVicente/Public domain.
In 1831, 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 utilised in the invention of the electric motor.
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.
In 1864, British natural philosopher James Clerk Maxwell combined Faraday's law of induction with three other equations: German mathematician Carl Friedrich Gauss' two laws concerning electric and magnetic fields, and French natural philosopher Andre-Marie Ampere's law relating magnetic fields to electric current.
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.
3.1 The electromagnetic spectrum ↑
British astronomer William Herschel 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.
German natural philosopher Johann Wilhelm Ritter predicted the existence of ultraviolet light in 1801, when he found that it reacts with silver chloride.
First electric street lights in Berlin, 1884 by Carl Saltzmann. Image credit: Carl Saltzmann/Public domain.
The electromagnetic spectrum was completed by the end of the 19th century, with German physicist Heinrich Hertz demonstrating the existence of radio waves and microwaves by 1888, German physicist Wilhelm Röntgen discovering X-rays in 1895, and French chemist Paul Villard discovering gamma rays in 1900.
By 1900, the light bulb, tram, and telephone had been invented, and the development of electrical transmission lines soon allowed the public to access electricity in their own homes.
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. If it exists, then the light of the Sun should be dragged forwards in its direction of travel, like sound in the wind.
This means that the speed of light should appear to be faster when it is travelling in the same direction as the aether, and slower when it is moving against it.
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.