7.2 Light

Critical Questions:

  • What is light made of?
  • Why is the sky blue?

The main reason that light has always been so hard to figure out is that it follows its own rules.

For example, we’re used to things having mass. People, ferrets, churches, very small rocks – all are made up of matter and thus do things like resist acceleration (according to Newton’s Second Law) and create gravitational fields.

Light, on the other hand, has no mass. It’s made up of… nothing, it would seem. And as a result, it neither exerts a gravitational pull on massive objects nor ever slows down.[1. Don’t get clever and tell me that light slows down when travelling through something other than a vaccuum, by the way – it’s not really slowing down, as I’ll explain eventually.]

We’re also used to dealing with waves. While solid objects run into each other and generally follow Newton’s Laws of motion, waves interfere with each other and generally don’t.

But when scientists developed the technology to look more closely at light, they found that sometimes it behaved like a wave – interfering with other light, for example – and sometimes it behaved like a particle, in that it seemed to come in countable numbers of separate objects.

Photons make their own rules, much like Steve McQueen's character in Bullitt.
Photons make their own rules, much like Steve McQueen’s character in Bullitt.

Once we started to make sense of all this, we had to assign light to a whole new category – neither wave nor particle but wave-particle, something which exhibits characteristics of both. And we eventually named the light wave-particle the photon.

Now, in order to explain another fact about photons, I’d like to step back for a second. You might recall from the section on Forces that the interactions between electrically charged particles (like protons and electrons) and the force pulling magnets together are, in fact, two varieties of the same thing: the electromagnetic force. Now, in order for two electrons to repel each other, for example, a signal has to pass between them. This signal is called an electromagnetic (EM) wave. Its exact nature is highly abstract and difficult to explain without a lengthy discussion of field theory, but you can imagine it as a little ripple of vibrating energy.

Well, it turns out that a photon is actually nothing more than a small ‘packet’ of electromagnetic energy, very similar to the signal sent between charged particles.[2. In fact, this signal is sometimes called a ‘virtual photon’, which is a somewhat inaccurate description.] This is a pretty strange idea, but it helps to explain a number of the things that happen at the subatomic level, which we’ll be taking about in Chapter 9.

But wait, we were talking about light. And at this point it may seem that this massless, fast-moving wave-particle/electromagnetic radiation has gotten fairly distant from what we think of as light. But in fact, we can already begin to explain a wide variety of everyday light-related phenomena using only the basics of this description.

For example, waves have a characteristic called frequency, and the frequency of visible light is what gives it its colour. And white light is not any one particular frequency, but is made up of all frequencies of light travelling together. This alone can help explain everything from why leaves are green to why the sky is blue.

As a wave of energy, light can be absorbed by the electrons inside atoms, in the same way that a buoy floating in water can absorb the energy of a passing water wave. But the difference is that every solid object can absorb only particular frequencies of light based on the patterns of its electrons[3. Yes, that deserves more explanation. I promise to get to it in Chapter 9.], while the rest just bounce off the surface. So, for example, leaves are green because they have quite a bit of a chemical called chlorophyll, which absorbs most colours of light but reflects green.

The answer to why the sky is blue is a bit more complicated, but it’s based on the same principle. The molecules in the Earth’s atmosphere are very good at scattering blue light in different directions, but they don’t affect the other colours as much. The sun emits all frequencies of visible light, but only the blue light gets scattered around. In other words, when you look at the sun it appears to be yellowish, which is essentially just white light with a bit of blue removed. When you look near the sun, you see the blue light that came from the sun and was scattered by the air and into your eyeballs.

Source: physics.org
Source: physics.org

What about sunsets? Well, when the sun is low in the sky, its light has to travel through much more atmosphere to get to your eyes. In that time, most of the light from the blue end of the visible spectrum has been scattered away, leaving a primarily red colour.

But here I am going on about “visible” light and the “visible spectrum”. Maybe you’re wondering whether there’s such a thing as invisible light. Well, there is. Remember how I mentioned somewhat casually that photons are really just little wave-packets of electromagnetic radiation, and that the frequency of that wave is the colour of the light? Well, the human eye can only see a certain range of frequencies of EM radiation. That means that any frequency outside that range is still, technically, light – but we can’t see it.

This category of invisible light includes the frequencies called “infrared”, meaning they have frequencies lower than red light. The amount of infrared radiation something emits is a good indicator of how hot it is, so it should be no surprise that most of the light coming from the sun is actually infrared. When the Predator from the movies sees in heat vision, he’s just using a machine that detects infrared light, then translates that into light in the visible spectrum. In fact, many snakes can detect infrared signals without the use of special goggles.

Or wait, did the Predator use goggles?
Or wait, did the Predator use goggles?

“Ultraviolet” (UV) light has a frequency greater than the violet end of the visible spectrum. Because of the nature of human skin, this stuff has the best chance of being absorbed and messing with DNA, resulting in skin cancer.

X-rays fall on the ultraviolet end of the spectrum, and they’re useful because they pass through most human tissue but are absorbed by bones and other solid materials. Radio waves, which can be found on the infrared side of things, pass through almost anything, so they tend to be used for sending signals through the air. The next time you use your phone, squint real hard and try to see the beams of invisible light it’s emitting! (Note: it is physically impossible to see cell phone signals.)

Source: pion.cz


All of this also fits with the fact that light tends to make things hotter. Since photons are travelling energy, it makes sense that when they bounce into things, those things tend to have more energy – specifically, vibrational kinetic energy, which we know as temperature (see Chapter 5!).

There’s quite a bit more we could say about light, but for now let’s leave it at that and move on to some practical applications.

Big Ideas:

  • Light is made of small, massless things called photons.
  • Photons are neither particles nor waves – they are “wave-particles”, things which have some characteristics of both.
  • They are also travelling packages of electromagnetic energy.
  • White light is made up of a combination of all colours (frequencies) of light. Humans can only see only colours within a certain range of frequencies.
  • If something has a specific colour, it’s because it emits (usually, scatters or reflects) that frequency of photon.

Next: 7.3 – Lenses, Mirrors, Cameras, and Telescopes

Previous: 7.1 – Introduction to Light and Optics