Visible light lies on a spectrum of electromagnetic radiation sandwiched between longer wavelengths like radio waves, microwaves, and infrared light, and shorter wavelengths like ultraviolet light, x-rays, and gamma rays. But there’s nothing about our everyday experience with light that would lead us to believe it has anything to do with electromagnetic forces. Indeed, it came somewhat as a surprise in 1864 when Maxwell calculated 1 the speed of theoretical electromagnetic waves and found that it was equal to the speed 2 of light! What’s more, these waves hadn’t even been demonstrated to exist! What experimental evidence led 19th century physicists to conclude that light and electromagnetic radiation were in fact the same phenomenon?
What are electromagnetic waves?
A wave is a disturbance that retains its shape as it propagates. A slightly slack piece of rope tied to a wall, for instance, can support waves: if you flick your wrist while holding one end, a kink will move down the length of the rope without changing shape. A sound wave is a moving pattern of high and low density in a medium—air, for instance. Compression waves like sound can also propagate along a spring:
Electromagnetic waves, not surprisingly, are moving disturbances in electric and magnetic fields. In physics a force field is really just an abstract description of how a particular force would act on object, regardless of whether it is actually there or not (this video is an excellent explanation of electromagnetic fields). It is visualized by hypothetically placing a “test” particle at a point and seeing what force the particle would feel. For example, a planet would gravitationally attract an object of mass one kilogram placed anywhere, and therefore we imagine a gravitational field pointing toward the planet filling the entire universe. A tray of iron filings over a magnet yields a picture of its magnetic field since each filing will align with the magnetic field at that point; to obtain a picture of the entire field, imagine filling all of space with iron filings.
Electric and magnetic forces are closely related but subtly different. An electric field is generated by any charged particle; it is responsible for the attraction felt by two oppositely charged particles. For example, electrons are tied to the nucleus in an atom, as well as a staticy balloon to your head, by the electric force. Magnetic fields are produced, on the other hand, by moving charges, and only act on moving charges. A charged wire generates an electric field but no magnetic field because none of the charges are moving. A wire conducting current generates a magnetic field (this is how an electromagnet works) but no electric field because there are just as many protons in the wire as electrons, so the net electric field is zero, but the protons are stationary and don’t generate a magnetic field.
An electromagnetic wave is a propagating variation in electric and magnetic fields in perpendicular directions. Importantly, there are no charged particles producing the field; it is self-propagating, because a changing electric field induces a magnetic field in a perpendicular direction, and vice versa (this induction is the principle by which a power converter works). Thus, there’s no such thing as a purely electric wave or a purely magnetic wave.
Conversely, an electromagnetic wave will not be affected by the presence of an electric or magnetic field because those fields only interact with charged particles. The key observation is that in the presence of matter, which is full of charged particles, it is possible for an electric or magnetic field to influence how the wave propagates through the matter because the fields affect the particles, and the particles in turn affect the wave. This is the essence of the Faraday and Kerr effects.
You can see in the above diagram that whenever an electromagnetic wave passes through a point, the electric and magnetic fields at that point are perpendicular but that doesn’t mean the electric field must always point along the same axis. If it does, the light is said to be plane polarized. This is in fact how you were able to watch Star Trek in 3D last week: polarized glasses only allow vertically polarized light through one eye and horizontally polarized light through the other, and by projecting two slightly different images in vertically and horizontally polarized light a stereoscopic effect is produced.
Light was observed to be polarized before it was understood what polarization meant. It had long been known that a crystal of calcite could split a beam of light and by passing one of those beams through a second crystal, the Dutch physicist Huygens (1672) showed that at some angles the second crystal did not split the beam (you can use a polarizing filter instead of the second crystal to isolate the two images). This is because the crystal refracts vertically polarized light one way and horizontally polarized light another, so if the second crystal is aligned with the first, the beam coming out of the first crystal will not be split by the second. You can produce a similar phenomenon by putting on 3D glasses from a movie, closing one eye, and looking in the mirror—one lens will be completely dark and the other completely transparent!
Some animals like insects, cephalopods, and a few amphibians can actually detect the polarization of light and use it for navigation—they have two sets of color receptors for different directions of polarization. Cephalopods can even control the polarization of light reflected from their skin, and may use it for communication! Even humans have a slight sensitivity to polarization, which you can see yourself in Haidinger’s brush.
The Faraday and Kerr effects
The first clue of light’s electromagnetic nature was provided by an experiment conducted by one of history’s greatest tinkerers, Michael Faraday, in 1845. He observed that by passing a beam of vertically polarized light through a transparent medium in the presence of a magnetic field, the direction of the polarization rotated in a measureable way.
The angle of rotation depends on the material and the strength of the magnetic field. Faraday originally did the experiment with glass, but you can achieve a rotation of a couple degrees with a small crystal of terbium gallium garnet and the electromagnet from a loudspeaker!
Slightly more difficult to measure, the Kerr effect demonstrates that an electric field affects the passage of light through materials as well. When a ray of light passes from one substance (like air) into another (like glass), the ray bends, or refracts; this is how lenses work. In 1875, the Scottish physicist John Kerr observed that the angle of refraction changes in the presence of an electric field!
Synthesis of electromagnetic radiation
The Faraday and Kerr effects are not conclusive because they only show that light interacts in some way with charged particles, although the fact that these effects are identical to those that electromagnetic radiation would experience (as calculated from Maxwell’s equations) is evidence in and of itself. And in a sense there is no one experiment that offers conclusive proof; only an accumulation of different circumstances under which light and electromagnetic radiation behave identically. Of course electromagnetic waves were still purely theoretical in 1875!
The existence of the waves predicted by Maxwell was proven definitively by Heinrich Hertz in a tour-de-force series of experiments published in 1887. He developed the first radio antenna, which essentially consisted of a loop of wire. An incident electromagnetic wave would move the electrons in the wire and produce a measurable current. The antenna was way too big to be able to see the field disturbance of visible light, but could easily detect radio waves. He then showed that a spark gap produced an electromagnetic wave whose periodic fluctuation in electric and magnetic fields could be detected by the antenna.
By scaling the same device down by a factor of roughly one billion, one could theoretically produce and detect electromagnetic radiation in the visible spectrum. Indeed, the wavelength of Hertz’s waves was about 4 meters, while that of visible light is about 6 nanometers, and to detect a wave an antenna must be roughly the length of the wavelength wave. But at the very least Hertz’s experiment produced electromagnetic radiation and proved it in fact did propagate at the speed of light, as predicted by Maxwell! Hertz actually demonstrated many more similarities to light, for instance that: (a) radio waves propagated in straight lines; (b) they can be reflected from a metal mirror; (c) they can be polarized; (d) they refract through a prism.
In the modern world, small versions of Hertz’s experiment actually exist all around us. Both microwave ovens and cell phones produce electromagnetic waves (in ovens at wavelengths of 120cm, and in phones at about 15cm) from oscillating electrons (in ovens with a magnetron, in phones with an antenna). Neon lights are perhaps the closest thing to a proof that visible light is electromagnetic radiation: visible light is produced by electrons jumping between energy levels in an atom, and this is essentially the same as an antenna of very small length.