Problems with Light (1800s)
German-British astronomer William Herschel was the first to show that light exists beyond the visible spectrum in 1800. He did this by measuring the temperature of different colours produced using prisms, an instrument popularised by Newton. Herschel found that the temperature was highest just beyond the colour red, leading him to predict the existence of infrared light (Ring, pp.800-802). A year later, German physicist Johann Wilhelm Ritter predicted the existence of ultraviolet light when he found that it reacts with silver chloride (Adorno, 1990).
In 1802, English physicist William Hyde Wollaston discovered dark lines in Newton's colour spectrum, which he believed to be gaps separating the colours. These lines were independently discovered by German physicist Joseph von Fraunhofer in 1814 and became known as Fraunhofer lines (Waldman, pp.156). Fraunhofer mounted a prism in front a small telescope to create a spectroscope, an instrument that separates light into a spectrum. With this new technology he was able to map over five hundred and seventy of these dark lines.
The Star Garden
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Fraunhofer lines in the Solar spectrum
In 1803, English physicist Thomas Young provided strong evidence for Dutch mathematician Christiaan Huygens' wave theory of light when he published the results of his double-slit experiments (Scheider, pp.217-219). Young repeated earlier experiments with diffraction but continued to pass the diffracted light through two more slits. He argued that if light is composed of particles, then they should all pass through separate holes and create two bright patterns on the other side. If light is composed of waves, however, then it should produce a predictable interference pattern, just as water waves do.
The double-slit experiment with particles The double-slit experiment with waves
Diffraction in water Photo credit: Fjellanger Wideroe Young's results
Just like water waves, two light waves that are emitted in the same phase create waves of twice their former amplitude. Whereas waves that are out of phase by 180 degrees will not have an amplitude at all, as the first wave is counteracted by the negative amplitude of the second. This is known as the superposition principle.
Wave in the same phase Wave out of phase by 180 degrees
In 1817, Young proposed that light waves have a transverse as well as longitudinal component and French physicist Augustin-Jean Fresnel made the same discovery independently that year (Waldman, pp.67). 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. The crystals act as polarisers, a term coined by French mathematician Etienne-Louis Malus in 1811, and this means that they only allow the ray to propagate in one plane, either horizontally or vertically. The horizontal component of the wave cannot travel through a vertically aligned polariser and the same is true the other way around. 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.
By 1845, English physicist Michael Faraday had discovered what is now known as the Faraday effect. 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 magnetic fields are somehow related to light waves. Faraday had previously shown that electric and magnetic fields are related by his law of electromagnetic induction. This states that a rotating magnetic field will create a voltage. Five years later, French physicist Leon Foucault verified Young and Fresnel's wave theory of light when he showed that light travels faster in air than in water.
In 1842, Austrian physicist Christian Doppler had discovered what is now known as the Doppler effect. This shows that a wave's apparent frequency will change, depending on whether it is moving towards you or away from you. This is evident in the sound of police or ambulance sirens but was first demonstrated with a train full of trumpeters in 1843. Doppler tried to apply his principle to light waves, but it was not until six years later that French physicist Armand Fizeau suggested that light changes wavelength, and therefore colour rather than frequency, as its velocity changes. Objects appear bluer when they move towards you and redder when they move away.
In 1861, German physicist Gustav Kirchhoff and German chemist Robert Bunsen related the placement of Fraunhofer lines to the absorption or emission of different chemical elements (Waldman, pp.157). Kirchhoff and Bunsen showed that Fraunhofer lines are caused by gases in the outer atmosphere of the Sun which absorb some of the Sun's light before it reaches us. By working out which lines correspond to which element, they were able to determine the composition of the atmospheres of different stars. The Doppler shift of the light of stars can be measured by comparing an objects emission or absorption spectrum with those measured in the laboratory.
Absorption spectrum
Kirchhoff and Bunsen showed that if a star is not present, then a hot gas will only emit light corresponding to the elements that it is made of, this results in an emission line spectrum.
Emission spectrum
Kirchhoff coined the term 'blackbody' to describe a hypothetical object that emits a continuous spectrum with no dark gaps. A blackbody absorbs all of the light that hits its surface, this means that it doesn't reflect light and it does not let light pass through it. When a blackbody is cold it is completely black and as it heats up it remains in thermal equilibrium, emitting light at all wavelengths. There is no such thing as a perfect blackbody but there are lots of objects that are close. Stars emit a continuous spectrum at their core and other approximate blackbodies include the filaments of light bulbs, the hob of an electric oven, larva and metals like iron.
Continuous spectrum
The relationship between a stars energy and temperature was not known until 1879 when Austrian physicist Joseph Stefan suggested that the flux of a blackbody - the energy it radiates per unit of surface area per unit of time - is proportional to the forth power of its temperature. Stefan's first PhD student, fellow Austrian physicist Ludwig Boltzmann, expanded this theory five years later and this allowed them to calculate how hot other stars are compared to the Sun. Stefan and Boltzmann were able to work out the temperature of the Sun's surface by comparing it to other blackbodies found on Earth. They estimated that the surface of the Sun is about 5700 degrees Kelvin (about 5430 degrees Celsius). This is only about 80 degrees less than the currently accepted value.
In 1893, German physicist Wilhelm Wien showed that the peak wavelength of a blackbody is only dependent upon its temperature, as it heats up a blackbody will move through the spectrum becoming red, yellow, green and then blue. The peak wavelength tells us nothing of what the blackbody made of (Dannon, 2005). By the end of the century, however, German physicists Heinrich Rubens and Ferdinand Kurlbaum would show that this theory does not apply to infrared light (Vazquez and Hanslmeier pp.6).
Three years after Kirchhoff and Bunsen proposed a link between light and the different chemical elements, Scottish physicist James Clerk Maxwell extended Faraday's findings by showing that light is connected to electricity and magnetism. In 1864, 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 mathematician 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. He proposed that light is a transverse wave composed of oscillating electric and magnetic fields (Maxwell, pp.459-512). Maxwell's electromagnetic theory, like all previous theories of light, was reliant upon the idea that space is filled with a substance known as the aether, a fact that was accepted for the next twenty three years.
The aether is a medium like air or water, 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. In 1887, American physicists Albert Michelson and Edward Morley devised an experiment which was precise enough to measure the difference in the speed of light as the Earth moves around the Sun. Almost everyone believed that the experiment would successfully prove the existence of the aether but it was a tremendous failure in this regard. Michelson and Morley found that the speed of light moves at the same rate in all directions (Michelson and Morley, pp.333-345). Not only did they disprove the existence of the aether, but they disproved all known theories of light.
Image credit: Wiki Commons
References
Adorno, T.W., 1990, 'The Form of the Phonograph Record', Levin, T.Y. (trans.), MIT Press, Cambridge
Dannon, V.H., 2005, 'Zero Point Energy: Planck Radiation Law', Gauge Institute Journal, Vol.1
Ring, E.F.J., 2004, 'The Historical Development of Thermal Imaging in Medicine', Rheumatology, Vol.43
Maxwell, J.C., 1865, 'A dynamical theory of the electromagnetic field', Philosophical Transactions of the Royal Society of London, Vol.155
Michelson, A. A. and Morley, E. W., 1887, 'On the Relative Motion of the Earth and the Luminiferous Ether', American Journal of Science, Vol.34
Scheider, W., 1986, 'Do the "Double Slit" Experiment the Way it Was Originally Done', The Physics Teacher, Vol.24
Vazquez, M. and Hanslmeier, A., 2006, 'Ultraviolet radiation in the solar system', Springer, New York
Waldman, G., 2002, 'Introduction to Light: The Physics of Light, Vision, and Color', Dover Publications, Mineola