QR3.2.1 Light is a Wave

Figure 3.5. The electromagnetic spectrum

Maxwell’s equations describe a photon as a wave in an electromagnetic field that sets imaginary values outside our space. If this wave vibrates slowly, we get radio waves, faster vibrations are visible light and very fast vibrations are x-rays or gamma rays (Figure 3.5). Visible light is the part of the spectrum that vibrates about a million-billion times a second, gamma rays are a billion times faster while radio waves vibrate just a few times a second. For simplicity, from now on the term “light” refers to any electromagnetic wave frequency.

In Newton’s optics, a light ray moves on an axis that can contain many photons polarized in different ways. Modern filters can polarize a ray one way and lasers can even produce a pulse of light of one frequency in one polarization plane on one axis, which is one photon.

When such techniques produce rays of polarized light that are out-of-phase, the crests of one match the troughs of the other. The result is two rays that are separately visible but combine to give darkness, as the out-of-phase photons cancel each other just as out-of-phase waves do. This light + light = darkness confirms that light really is a wave as particles can’t do this. Note that flashlight beams can’t do this because they aren’t polarized.

Figure 3.6. A circle maps to a sine wave

We also know the type of wave. Light is a sine wave that in mathematics maps to an extended circle (Figure 3.6), so a pointer turning in a circle like a clock hand can describe a sine wave, as shown in Figure 3.7.

Figure 3.7. A sine wave is a moving rotation

Wave theory describes a water wave as a sine wave caused by the forces of gravity and elasticity acting at right angles to the water surface. When the wave arrives, a surface water molecule is pushed say up until gravity pulls it back down, then the water elasticity pushes it back up, etc. The wave just moves water molecules up and down so corks just bob up and down as a wave passes. What travels on the surface is a transverse vibration not the water itself.

We describe light in the same way but call it an imaginary wave because no-one can say what is going up and down. Naming a cause doesn’t explain it, so the term electromagnetic field is just a placeholder for what we don’t understand. Yet if light is a wave, it must vibrate on space, and this is something we find difficult to imagine.