6.2 Waves

Critical Questions:

• What is a wave?
• How does a wave travel?

There’s a story I heard somewhere – I have no idea whether it’s true or not, but I like it anyway. The story is that a group of people somewhere in the world developed a language that had an interesting way of referring to water waves. Instead of pointing at the ocean and saying, “Hey look, there’s a wave,” they would point at the ocean and say, “There is waving.”

If this is a true story, two things are possible: the first is that these people disliked the expression “Hey look,” and the other is that the speakers of this language understood wave behaviour much better than most.

The reason “waving” is more appropriate than “a wave” is that waves are best understood not as objects but as actions. A water wave, for example, isn’t a thing in many senses of the word – it is definitely not solid, it may have a beginning but often lacks a clearly definable end, and it can’t be picked up and separated from the water it’s made of.

When you look at a water wave, what you are seeing is the combined motion of billions and billions of individual water particles. These particles – molecules, in this case – are all connected by weak electrical interactions called hydrogen bonds. They form an immense molecular web that stretches off in all three dimensions. When you jostle them around, they push and pull on their neighbours, and they may just bounce back to where they started.

A nice way to imagine this is to get a few of your friends to stand side by side in a line, holding their hands out in front of them. String a length of rope along their hands, and tell them to hold tight. Now tell the person on the far left to lift up her arms. If your friends are good sports, you’ll see their arms all lift up, one by one, starting from one end. This is a wave; the hands are the water, and the rope represents the chemical bonds.

If they can get a good rhythm going, this should look a lot like fans doing The Wave in a sports stadium. And if you watch this for long enough, you may come to a rather startling insight: although the wave itself is moving from left to right, each individual particle is only moving up and down.

You can test this observation by running a bath[1. Baths are always a good idea, but they’re even better if they’re for the benefit of science.], letting the water settle down, and then dropping a rubber duck into it. Push the palm of your hand gently up and down into the water nearby – if you’re careful, you’ll see the duck bobbing in place rather than moving with the waves.

This is one of the basic principles of wave motion: a wave moves through a medium, causing the particles in that medium to vibrate but not to travel.

This is the main reason why waves can travel so quickly. Sound waves, for example, move through the air at over 1200 km/h (760 mph), which is the reason why you can hear a far-off plane long before it disappears over the horizon. A sound wave is just vibrating air particles; if the only way to carry sound were to push air around from one place to another, it would be very slow – the fastest wind speed ever recorded was only about 400 km/h (250 mph).

This also explains why sound travels more quickly in more dense media. Sound can travel through water at around 1500 km/h (930 mph) because the water molecules are more tightly linked than air molecules.

I should point out that there is a major difference between sound waves and regular water waves: in a sound wave, the air particles bounce back and forth rather than up and down, as in the mesmerizing image below:

What’s happening here is that something like a drum skin on the left is moving the particles around it. They rush off to the right, then bump into other particles and return to the drum. This causes the next particles to move forwards, and so on until the wave has passed all the way to the right.

In a water wave, meanwhile, the particles tend to vibrate in elliptical loops:

However, in real life, things are always a bit more complicated. For example, the waves crashing into a beach are certainly moving water fairly large distances, right? A surfer doesn’t get up on his board and make little loops like in the image above, does he?

The reason these waves are different is that they break, which the animated waves above do not do. When a wave breaks, its whole shape gets changed. The water beneath the wave gets slowed down by the sea floor, which rises as the wave approaches land. At some point, the energy of the wave moves the water on top much more quickly than the water on the bottom, and the top-water spills out over the wave and gets pushed along by the wave’s energy.

But breaking waves aside, having this basic idea of waves as particle vibrations passing through a medium can be extremely helpful. As we’ll see in the next few posts, this will allow us to understand almost everything about waves, from musical sounds to the functioning of the human eye.

Big Ideas:

• Waves are made up of vibrating particles.
• When a wave travels, the particles of the medium do not travel with it.

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Next: 6.3 – Sound

Previous: 6.1 – Introduction to Waves and Sound