How to Measure the Speed of Light with Marshmallows – Christmas Lectures with Neil Johnson


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]