UK

035

14 India Street, Edinburgh, Scotland

55° 57′ 19.17″ N, 3° 12′ 19.78″ W

James Clerk Maxwell

Scotland has produced a great number of famous scientists and inventors, including Alexander Graham Bell (Chapter 4), Lord Kelvin (Chapter 73), John Napier (Chapter 57), John Logie Baird (page 452), and James Watt (page 299). But in the world of mathematics and physics, one name stands above them all: James Clerk Maxwell.

Einstein described Maxwell’s contributions as “the most profound and the most fruitful that physics has experienced since the time of Newton” because in 1864 Maxwell showed, in the paper A Dynamical Theory of the Electromagnetic Field, that light is actually formed from electromagnetic waves. He also suggested that there might be other types of radiation obeying the same laws, and it wasn’t long before other types of radiation were discovered: radio waves were found by Hertz in 1886, X-rays by Röntgen in 1895, and gamma rays by Villard in 1900.

And, above all, Maxwell’s important theoretical step underpins Einstein’s 1905 work on relativity. But completely changing physics wasn’t enough for Maxwell’s prodigious talent—he also made a major contribution to thermodynamics and the kinetic theory of gases.

Since Maxwell’s advance was entirely theoretical, there aren’t any inventions or apparatus to visit. There is, however, the James Clerk Maxwell Foundation, which is headquartered in Maxwell’s family home on India Street in Edinburgh.

The building is home to the Foundation and shared with the International Centre for Mathematical Sciences, but its interior is filled with objects related to James Clerk Maxwell, his family, and his work. And there are examples of one of Maxwell’s little-known talents—writing poetry.

The house itself and its location attest to the fact that Maxwell’s parents were well off. His father was an advocate (similar to a lawyer), but luckily allowed his son to forgo following in his footsteps and let him study at Edinburgh and Cambridge universities.

Maxwell also took what is considered to be the first color photograph. Working with a photographer, he took a picture of a piece of tartan ribbon tied into a bow using a simple camera and three color filters (one each of red, green, and blue). He then projected the three images on top of one another to reproduce the original scene. The photographic plates he used are on display at 14 India Street. To this day, computer and television screens reproduce images using mixtures of red, green, and blue light.

Practical Information

You will find the website of the James Clerk Maxwell Foundation at http://www.clerkmaxwellfoundation.org/. Note that you must make an appointment to visit the house, as it is not open to the public for unscheduled visits. Visits are free, but it’s good karma to make a donation to keep the foundation alive.

Waves and Particles

It would be nice to have a thorough discussion of Maxwell’s equations in this book, but to really do them justice we’d need another 50 pages (Maxwell’s 1864 paper runs to 54 pages of dense theory and mathematics) and the willingness to navigate a minefield of partial differential equations over vectors. If you are interested in the details, the HyperPhysics website is a good starting point.

But even without the mathematical detail, the result of Maxwell’s equations—that light and other radiation are electromagnetic waves—is the foundation for much of modern life. And it was only half right: light behaves as both a wave and a particle.

Maxwell’s calculations showed that light was formed from a pair of waves, at right angles to each other (Figure 35-1). One wave is formed from an electric field that varies in amplitude (i.e., its strength varies), and the other wave is a varying magnetic field. In fact, the electric wave creates the magnetic wave and vice versa. The two waves are in phase with each other and have the same wavelength (the distance between two peaks of the wave).

Figure 35-1. A light wave

Different wavelengths cause electromagnetic waves to have different properties (see Figure 35-2). Visible light has a wavelength of between 400 and 700 nanometers; blue light has the shortest wavelength, and red the longest. To one side of the visible light spectrum are ultraviolet waves (roughly between 200 and 400 nanometers), which are responsible for sunburn, and on the other side is infrared (between about 700 nanometers and 1 millimeter), which is used for night vision, remote controls, and thermal imaging (where you can see the heat being given off by an object).

Figure 35-2. The electromagnetic spectrum

X-rays and gamma rays have wavelengths shorter than ultraviolet; microwaves and radio waves sit to the other side of visible light with wavelengths longer than infrared. Normally when talking about radio waves, the frequency of the waves is given (e.g., a radio station might be announced as KQED 88.5 FM, where 88.5 is the frequency in MHz). The frequency, f, is just the number of peaks of the wave per second. For electromagnetic waves (which move at the speed of light, c), frequency is related to the wavelength, λ, by the formula in Equation 35-1.

Equation 35-1. Wavelength and frequency

But Maxwell’s equations did not answer all the questions about radiation. In particular, the photoelectric effect—where electromagnetic radiation is absorbed by matter, causing the emission of electrons—could not be explained from the wave theory. Maxwell’s equations predicted that this release of energy (in the form of electrons) should be proportional to the intensity of the electromagnetic radiation, but it was actually proportional to the frequency. The effect was well known—in 1901 Nikola Tesla (Chapter 30) applied for a patent for a device that charged a capacitor from an electric plate exposed to sunlight.

The missing link was discovered by Einstein in one of his Annus Mirabilis papers in 1905 (see also Chapter 33). He showed that light also behaves as discrete packets of energy (now called photons) and that the energy, E, of these “quanta” of light was proportional to the light’s frequency, f (see Equation 35-2). This went a long way toward explaining the photoelectric effect—light hitting a piece of matter was being absorbed in discrete units (photons) with specific energy related to the light’s frequency, and this energy was sufficient to knock an electron out of its orbit around an atomic nucleus.

Equation 35-2. Energy and frequency (h is Planck’s constant)

More electrons can be forced out of a piece of matter by increasing the light’s intensity, but they will all have the same energy determined by light’s frequency. The energy of each photon is partially used to detach the electron, and the remainder turns into the kinetic energy of the photon itself.

Maxwell introduced the electromagnetic wave explanation for light and other radiation, and Einstein showed that in fact the same radiation also behaves as if it were discrete units of energy, like particles. These two apparently opposing views are not contradictory—they are the basis for the wave-particle duality of matter, which says that all matter behaves as both waves and particles.

This duality has been experimentally determined for particles other than photons. Because the wave theory has the simple result of explaining diffraction and diffraction patterns, physicists have looked for diffraction patterns caused by other particles.

The 1937 Nobel Prize in Physics was awarded for the successful detection of diffraction of electrons (thus verifying that electrons have wave-like properties). In the electron diffraction experiment, a beam of electrons was directed at a nickel crystal. A movable detector (a galvanometer) was used to measure the resulting spread of electrons around the crystal. By looking at the peaks and troughs in the diffraction pattern caused by the crystal, the wavelength of the electrons was determined.