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Monday 6 March 2017

Answers for students: What links electromagnetic waves and ickle fluffy ducks?

This post is part of a series answering some of the physics questions students ask me most often.

What do waves have to do with radiation? 
the short answer is 'everything', but that wont help in an exam. 

The long answer?

Well.... look at this bridge.....

Above: Watch a bridge fall down because of standing waves. Go on, everyone likes seeing big thngs fall down.

Did you watch up to the bit where the man smoking the pipe rescues the dog? 
How about the bit where the bridge fell down? That's the important bit (although the dog rescuer was my favourite bit). 

This is what happens when you build a bridge without taking into account the way strong winds can resonate with the structure and cause standing waves: The wind blows just right one day and....

But I'm getting ahead of myself: Before we can collapse bridges with waves* we need figure what waves actually are (and how they relate to radiation)....

So let's start smaller, with a cat in a boat on a really still lake (stay with me, this will go somewhere). The cat is watching a tasty looking duckling swimming a meter away from the boat. Luckily for the duckling cats don't much like water, and all the cat can do is pace frustratedly back and forth inside the boat..

Frustration thy name is hungry cat in a boat watching a passing duckling. Boy you have a long name, stick to frustration. Image by K.J. Rogers

The cat's pacing makes the boat bob up and down, sending waves towards the duckling. When the waves hit the duckling they make it bob up and down. 

This is the odd bit: Obviously the boat bobbing up and down has moved the duckling - but I water has passed from the boat to the duckling.

Think about that: Even though the waves have moved from boat to duckling, the water hasn't: If you shot the duckling with a water pistol then (I'm not suggesting that's the kind of thing you'd do, although you could be anyone, so you might) the force of the water would push the duckling sideways.....

Really? Could you really shoot the ickle fluffy duckling? If you could, have you considered a career as a London city banker?

.....but when the waves hit the duck it just moves up and down -  so the waves haven't actually carried any water sideways from the boat**, they've just carried the up and down motion.

If that's not too clear to you (and, for a long time it was as clear as mud to me), watch these ducks, and see how their position relative to the shore hardly changes as the waves strike them - they move up and down a lot but hardly move towards the shore at all, even though the waves are definitely moving that way:  

Above: Ducks bobbing in the waves - ducks are more insightful than you might think.

So, if the water hasn't travelled, then what is it that has moved from the boat and made the duckling bob up and down? 

The answer is in the question: The duck has moved up and down, so what has travelled from the boat must be the up and down motion (called an oscillation) itself - and it has done that independently of what direction the water is moving (or not moving)! A motion that travels without the thing it's moving in travelling with it seems like an odd concept - the bounce on a bouncing ball doesn't run off to the shops on its own - but if you've ever been part of a Mexican wave at a football match you've taken part in exactly the same thing: Each person just stands up and sits down a moment after the person next to them does - they don't run around jumping up and down. Yet the overall effect is definitely a moving wave:

Above: A huge Mexican wave. If you're gonna put a Mexican wave in your blog, go big or go home, that's what I've always said. But I am a bit odd.

With a bit of deduction we can go further, and suss out something general about waves: To make the duckling move takes energy, like any motion takes, so the wave must be moving energy from the boat to the duckling. 

And that's the key thing that tells us what all waves really are: They're down to energy being transmitted through whatever medium the wave is in, as an oscillation (of some kind) without actually moving that medium itself (or at least not moving it in the direction the wave is going). 

What does that have to do with radiation?

Of all the kinds of radiation there are (and the word radiation is often used to include fast moving particles) they're all down to energy being transferred from one place to another - and to do this nature uses moving oscillations - waves! For electromagnetic radiation - which means radio waves, microwaves, terahertz waves, infra red waves, visible light, UV waves, X-rays and gamma rays - the waves are oscillations in electric and magnetic fields.  Other kinds of waves (like sound waves) aren't often called 'radiation', but they still work on the same principles, and 'radiate' energy away from their source. Recently you may well have heard of newly discovered 'gravitational waves' which are a moving oscillation in space itself.

But, sticking to electromagnetic radiation for now: what type you have depends on its wavelength and frequency (the two are related, and we'll get to that in a later post), and if you arrange the different electromagnetic radiations by wavelength you'll get an electromagnetic spectrum. 

Above: The EM spectrum. Yes, visible light is just that tiny little bit of it. Yes, that does mean you can't see 99% of what goes on in the universe. Yes that's a perfect excuse to ask for infra red night vision for next Christmas. Ask Santa to get me a set too, while you're at it. Courtesy of Lumanix.

The colour of visible light depends on its wavelength, and if you keep dialling the wavelength up or down you move to a different kind of radiation.... so the differences in types of EM radiation can be thought of as really extreme kinds of colours - too extreme for our eyes to see. For example: the human eye sees light with  a wavelength of 660 nanometers as red (one nano meter = 0.000000001 meters), and 550 nanometers as green. 350 Nanometers is UV light, too short to be seen by the human eye (bees can see it though) but detectable to our skin, which turns tanned in response to it. 1500 Nanometers is infra red, too long to be seen, but we can sense it as heat (and some snakes, fish, and predator can sense it well enough to hunt by it).

Above: The main features of a simple wave, sometimes called a sine wave - because it's an idealised wave that matches very closely to the sine and cosine waves you've probably learned about in Maths.

So all waves, across the universe, are nature's way of moving energy about without having to move matter. Radiation (or at least one very common type of it called electromagnetic radiation) is made of waves moving through electrical and magnetic fields

Suggested activity:

Feel like investigating waves in class? This is a way of making nice controllable waves out of sweets. Science you can eat!


1: Why does a cork on the sea bob up and down with the waves, but not move sideways or forwards/backwards with them?

2: When a wave is generated by something moving, what is it that the wave is carrying away from the movement that generated it?

3:  Light and radio waves are closely related. State two similarities, and one difference between them.

Answers here.

* A good physicist is 50% supervillian

** If you get really big waves, or waves breaking on the shore, things are more complicated, but that's a discussion for another post.

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