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. 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. 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. 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]