So we've said a lot about light.
It turns out the light is actually only one
of the waves in the same spectrum.
The electromagnetic spectrum.
Well what else is in that spectrum?
I have here my model which shows us.
Here we have light in the centre.
This is frequency going along here.
Number of oscillations per second.
As we go up in frequency, we get ultraviolet.
Well, that's what makes us go brown
or in my case red in the sun.
And if I go up further in frequency, we see x-rays.
Very high frequency, very short pulses,
can see things within the body.
Still further we have cosmic rays and gamma rays.
If I go down in frequency, we have infrared--
which actually we know is heat--
and radio waves.
So television waves.
The waves from mobile phones.
They're all part of the same spectrum.
They're all the same waves.
They all travel at the same speed.
And that speed is the speed of light.
That speed's very special.
I'll tell you a bit about that in a moment.
But first of all, we're going to have a go at measuring
the speed of light.
And I need a volunteer to help me.
Could you come up please?
And your name is?
Helen.
Could you come and stand here, Helen,
while they bring on the speed of light measuring equipment?
That's it.
Just stand out of the way.
Helen, just come around this side.
Because, Helen, you're going to be data collector.
This is the data we need.
We need the wavelength and the frequency of the light
because we need to multiply these two together.
Are you good at multiplying, Helen?
Not too bad.
OK.
We always got the calculator here.
That will give the speed.
OK?
So let's start measuring the wavelength.
What do we need to measure the wavelength of light?
A microwave and a dish of marshmallows.
And here they are arranged in a line.
And we're going to bung them in the microwave and cook them.
OK?
So let's just close the door.
Now, don't try this at home.
Now what's going to happen to these marshmallows?
Well they're going to cook, but they're actually
not going to cook evenly.
Microwave ovens, particularly if you've turned off the rotating
table, don't cook evenly.
And what happens is if you remember back to the wave
machine when there was a wall and we
sent a wave against the wall, we set up a standing wave.
A wave that didn't move anywhere.
It just stood still.
Well that's our key for measuring the wavelength.
Because in the points where the wave is actually
moving up and down a lot, it has a lot of energy there.
And so there the marshmallows should actually melt first.
So let's have a look, Helen.
Let's take out these marshmallows.
And yes.
We have a look at them.
You see some of them are melted, others haven't.
Then they've melted, others haven't.
There's actually a pattern.
So just like on this card here.
If we take this point is the point where
the marshmallows have melted and this point,
it will actually give us a measure of half the wavelength.
So let's do that.
Helen.
Let's close that door.
Let's take out a tape measure.
I'll have a ruler here.
And Helen, if you'd just like to help me measure the distance.
We need the separation of these two melted bits.
About 6.
OK.
So that's half of the wavelength.
So 6 centimetres.
So Helen, if you could write down here
12 centimetres as the wavelength.
That's right.
Just write centimetres here.
That's great.
Now we need the frequency.
Frequency is on the back.
Come around and have a look with me.
Come and have a look here.
It says 2450 megahertz.
Now megahertz is a million hertz.
A million oscillations per second.
So let's write down 2450.
Now we need to put a million after it.
So it's six zeros.
That's great.
Now we need to just multiply these things together.
Except we should really convert this
into metres because this is oscillations per second times--
we'd like it to be metres.
So let's turn 12 centimetres into 0.12 metres.
And we're going to multiply these two numbers together.
Well, let's put an equal sign here.
And write down here 6 zeros.
So we take care of the zeros.
And now I want you, Helen, to multiply 0.12 by 2450.
That gives 294.
So let's put in some commas here because I can never
rate these things with all those zeros.
There we go.
So what we've got is 294 million metres per second.
And here we have the number that physicists
use for the speed of light.
And it is 300 million metres per second.
So you've measured the speed of light.
Thank you very much, Helen.
[APPLAUSE]