And somebody wanted to know whether double rainbows were the same kind of effect.

So I thought I'd talk about repose.

So really, to explain this, I need to do an experiment.

I need to show you what?

A rainbow, Actually, yes.

So I have here the sun and a raindrop not to scale.

I have to say on.

So hopefully with these two fine tools, we should be able to demonstrate the physics of Rambos to switch the lights out.

Okay, so if you kind of come down to my level, okay, so if you work your way around towards the right, there's a kind of a little you can see the little white dot That sort of reflected off the back of our giant raindrop there.

And as you go around, you will see that it kind of moves towards the rights that kind of comes towards you.

And then at some point, suddenly it turns color.

So you'll see it go blue, green, red, and then disappear entirely.

And the point is, you're currently 42 degrees away from the direction that the light shining in from Europe.

Okay, so what we've got is we got the light coming in, and, as you can see as it passes through this surface so it goes from one medium to another, the speed of light changes, which has this effect of changing the direction that the lights traveling in.

But the amount by which it gets changed depends on the wavelength of the light blue.

Like it's been a little more than red light, so you can see that the blue light reaches the back of the drop in a slightly different point from the red light.

And then when they get reflected back again and again, there's another bit of refraction when they come out the raindrop again on you can see that basically the light, the red light, at least in these extreme raise, ends up traveling slightly, slightly wider angle than the boom.

That's when when you, as you were panning around, the last light you saw was the red light.

So you see just normal reflection pretty much all the way around to here.

And then when you wrote this last point, what's last angle.

That stuff could actually get through the drop it all.

You first see blue light, then you see red light.

Then you see nothing further out.

Of course, that's not what happens with the rainbow with a rainbow.

What?

You have another picture?

Because you have lots and lots of drops, which means there's always one of the right exactly the right angle to send the red light towards you or to send the blue light towards you.

And so, actually, what you end up seeing is that kind of remember this this at magic angle of around 42 degrees, anything that's around 42 degrees that this angle here between the direction the sun roof raiser coming from and the direction you're looking from, you'll end up seeing either red light or a little bit further out.

You'll see blue light.

That actually means that Rainbow is actually are around.

What, this actually, this a circle of points of the 42 degrees away from this line to the sun.

But what actually happens is that half of that circle is typically below the horizon, so that you see like half a circle of the of the light, and so you'll end up just seeing the classic kind of rainbow semi circle.

If you have to be at this magic 42 degree angle, how come often see a rainbow and Johnny Smith, five miles away, because he's looking at difference at a raindrop.

So he's still looking 42 degrees away from the sun.

But from where he's standing, he sees a completely different set of raindrops that are all doing exactly the same thing back towards you.

So when I see a Rambo at my house and you see a Rambo over at your house, we're actually seeing two different rainbow.

You're seeing a different set of raindrops that are causing the effect.

I mean, obviously the same son that's causing the effect, and it's the same physics.

But it's a different set of raindrops reflecting it back.

The and the other thing you notice is that when you went beyond so when you were looking kind of closer to the sun, so closer to straight, backwards and forwards, so kind of when you're looking at it, like just reflected off the back of a raindrop you saw basically white light.

And it's only when you get to these these extreme angles that you actually see the red and the blue light because that's why if you look at a picture of a rainbow, you always see the inside the rainbow.

He's always brighter than outside the rainbow, because inside the rainbow you're seeing mostly that white ish light being reflected back.

It's the sum of all the different colors being reflected back to you.

So you see, it is white light.

Then you reach this age of it where you're just about seeing just those last few colors, and then when you go to tow a wider angle, still you see nothing at all.

And that's why the outside of the rainbow appears darker than the inside at the stuff I thought you were holding out.

The promise of something about double rainbow Double Rainbow is weakened due to so double rainbows also again classically understood, going back at least two faras, so Day card did this experiment again in the 17th century.

Lost people had a go at this, and what they realized is so if we go back to our original picture for just a second.

So what was happening here is the light comes in, gets refracted, reflected.

Once off, the back of a raindrop comes back out refracted again in a double rainbow.

What happens is the light takes a slightly different path.

Another picture you can have the light coming in, gets refracted, gets reflected once reflected twice and then comes back out again and comes out at a different angle from if you had just undergone a single reflection of the bank.

This is another way of making a rainbow a different angle.

So the first way still works.

You can have things like coming in and get reflected once, and that'll give us a rainbow one angle or it could go in and get reflected twice.

And then we end up with a rainbow two angles.

So you end up with a picture like this.

So we still got the first rainbow here, where the light's coming in, getting reflected back to us just once off the back of the raindrop on.

So we should see the colors spread blue to rid.

But there's a second way that it can happen, that it can actually bounce twice around the raindrop.

Take this rather more complicated path and What you can see is that actually the rainbow ends up being reversed because of that second reflection where the red light was originally on the outside.

If you look at a second rainbow, you'll see that actually the color is inverted that the original rainbow The inner rainbow goes from blue to red The second rainbow goes from red to blue and in fact a ll the effects of reversed.

So as we were talking about before you see a brighter light down here, then you see a normal rainbow for the other rainbow.

Everything's reversed so you see the second rainbow and then you see brighter light further away.

And so the famously you see this thing called Alexander's band which is the dark region in between, where there's no white light gets reflected at all.

Because in the first rainbow it's all ending up down here and from the second rainbow, it's all ending up there.

It seems to me we should see a double rainbow every time and yet we only occasionally see a double rainbow.

It really is because that so what happens is that in the rain drop most of the light just go straight through, only a little bit of it gets reflected.

And so, actually, it's a very inefficient process.

Most of the light just traveled straight through the rain drop.

A little bit of it gets reflected.

Of course, if you want to affect it twice, that means a little bit gets reflected, and then only a little bit of the little bit gets reflected.

And so the second rainbow is always much fainter than the first.

So it is actually always there when in fact a very occasional you can even see 1/3 rainbow cheese even think to.

Still, So these are the rainbow is always still there, but they're always much fainter.

So if you can only just see the primary rainbow, you're not going to see the second.

Remember little.

It's always when you have a really stalking the bright primary rainbow that you end up seeing.

The second people often ask this question.

You know that you lose the magic of something by actually understanding it by actually understanding where a rainbow comes from, isn't it?

You're not beautiful anymore, because something you've explained it.

It's all very simple and trivial.

And of course it doesn't I mean, you know, I understand the physics of Rainbow is, but I could still look, go out, Look at once like, Wow, that's pretty.

So it really did you know, it doesn't have that negative effect at all.

It's just it just adds to the experience of seeing a rainbow that not only does it look beautiful, but you actually understand why it looks beautiful too.