That's the sun, a nuclear fusion reactor at a temperature
of several millions of degrees in the centre but, fortunately,
somewhat less on the outside, but still 6,000 degrees as I
have here.
The sunbeams come 93 million miles to us
in eight and a half minutes and arrive at the outside
of the atmosphere--
will you just hang onto that one Jeanne?
Thank you.
I'll take away this so it's less confusing--
arrive at the outside of the atmosphere
in their pristine state looking somewhat like that.
But by the time they arrive at the surface of the earth,
they are pretty tattered.
They look like that.
They've lost perhaps half of their energy,
and there are holes here in the infrared.
And I want to ask-- that's the real sunbeam which we see,
and I want to ask why that is.
What happens to the poor old sunbeam as it comes towards us?
Well, of course, it goes through the atmosphere.
I haven't had an atmosphere on here before.
And if I did have the atmosphere,
you wouldn't see it really because it would only
be 2 millimetres thick.
So let's blow up a little bit here so that we can see better
what we're doing and put this sky, the atmosphere,
over the earth.
Now to get him straight, Jeanne, pull it down a little bit.
Right, and now let's put on a sunbeam coming from our sun
here.
Oh, yes, Jeanne wants us to have a few clouds.
But let's have our sunbeam coming
through a nice, clear blue sky because, otherwise,
what I have to say gets--
well, we'll deal with clouds later.
But there we are.
That's what we're talking about.
Now, what I've shown here is the spectrum of all the colours.
And I've put the infrared on as well and the ultraviolet.
I didn't know what colour to put those because you can't see
them anyway.
They're represented by these waves here.
Now, what I want to ask is, why, as it
comes through the atmosphere, does the sunbeam get tattered?
And there are two reasons for this.
Partly, the light is scattered.
And partly, the light is absorbed.
And let's talk about those in turn.
If I have a bit of white card and I shine white light on it,
it's the light is scattered, isn't it?
If I have little white bits of paper, it's scattered.
If I have even drops of water, it's scattered.
Here I have some drops of water, a boiling kettle,
and rather like a cloud, condensed drops of liquid water
forming a cloud.
Now I should switch the light on,
and you can see the scattering of the light.
And it's white because those particles of steam
are fairly big.
And they just reflect like paper.
But now if I make some smoke here,
ammonium chloride, which has much smaller particles, watch.
What's the difference between the two?
This, yes, this is blue, isn't it?
That's white, and this is blue.
Now that is why the sky is blue because the sky is made up
of an atmosphere of very, very little particles, air
molecules.
And the light, the blue light, because it's
a short wavelength, is scattered better.
The particles are smaller than the wavelength of light.
And the blue light is scattered better than the red light.
It interacts more.
And it was Professor Tyndall working here in the Royal--
he was the resident professor here in the Royal Institution
after Faraday.
And it was he who studied scattering and to explain
the blue of the sky.
And if you explain the blue of the sky,
you'll get a bonus because you explain the red of the setting
sun as well.
Let me show you a setting sun and some blue sky.
This is a tube used by Tyndall, here.
And we're going to shine a light through the tube.
And then at the moment, you will see the light is going straight
through here.
OK.
And Mr. Smoke is going to--
going to Coates.
Mr. Coates is going to smoke.
He enjoys this experiment.
It comes in the middle of the lecture.
And he's going to put some smoke in this tube.
And I want you to see how the blue scattering occurs, first
of all.
And just put there.
You see the smoke.
That's enough, Bill.
You see how the blue scattering occurs there.
Can you see that?
It's blue, isn't it?
But you see how it gets red as it gets beyond here.
And that's because all the blue is scattered,
and so there's only red left to scatter.
Well, now if we go through a longer atmosphere
as we do when the sun sets, it will become redder and redder.
And now watch the sun as we put more and more smoke in.
Here's the sun.
And now we put smoke in.
You see it's quite white at the moment.
And we're scattering blue light from the smoke.
Now it goes red.
As it goes through more and more atmosphere
and we get more smoke in still, it
gets redder and redder until he's set.
Right, thank you, Bill.
And that--
[APPLAUSE]