But actually, what I'm gonna show you first is this beautiful crystal crystal of calcite That is a big Krista.

It's an enormous crystal.

It's actually just calcium carbonate.

So it's the same stuff that limestone and marble it made off.

But just in some places in the world, it just forms into these perfect crystals of calcite.

It's kind, of course.

I called Iceland sparkles, I think the first those founding in Iceland.

So if you just focus on the square there already Yeah.

Okay, now, if I put the crystal in there, you can see that the images instantly becomes doubled.

It's a strange property of this particular type of crystal that it creates kind of double images of things.

But there's another even stranger property, which is that these are now the individual images reach polarized.

And I can show you that by sticking this year.

This is just a regular sheet of Polaroid.

And if I get this just right so some angles, we see both images, but I rotated around.

Sometimes you'll see one image as we keep going around.

Sometimes you see the other image come back the other way again between one image in the other.

And that's because one of the images the light is polarized in one direction and the other image delights polarized in the other direction.

Your face to double happen if it doesn't work so well, yes, where you wanted to see you try and do The thing with total weight is well, let's say we could do that.

I just rotated around.

Yeah, it works.

Cool.

You can explain it.

Okay, you need to sit down for this.

Okay, so it's a single buyer for engines and it's to do with refraction, which is the property of material.

Every material has a refractive index, and it's basically is to do with the speed of light at the speed of light.

In a vacuum is always the same 300,000 kilometres per second, but the speed of light through different materials is different.

The basically the speed of light slows down for materials and essentially is because, as the lights traveling through lights, this electromagnetic way that's wiggling around the electric fields of wiggling around that makes the electrons in the in the material wiggle around as well, and as they wiggle around, they produce electromagnetic waves of their own, which kind of combined with the light that was already passing through the net effect of all that is to make the light as it travels through slow down.

We use this all the time.

I mean, it's why lenses work while Lindsay's been light and so on and the reason why.

So if we have to go back to our crystal here right now, we gotta lie approaching this crystal as it approaches.

Of course, if it's not, if it's face on the tunnel, the light hits at the same time.

But if it's coming in at an angle, then some of the light hits first and then the rest of the wave hits later on.

So as this first bit of light hits, it slows down.

But this bitch up here that hasn't hit the service yet, still going at the same speed.

The net effect of that.

It's like if you were to drive through a puddle in your car, right?

Only the puddle only hit one set of wheels.

What tends to happen is the wheels on that side slow down and you'll find your car tends to try and turn that way.

So it's just the same with light.

The light it's here.

The bit that hits first slows down on the Net.

Effect of that is that it turns the direction of the light.

So one of the effects of glass of light hitting the surface like this where there's a refractive index in this material is it changes the direction in which lights going.

Quite that's this effect called refraction.

And that's how prisons work.

And Lindsay's working all sorts of things like that.

So that's what that's what well known phenomenon.

The interesting thing about this particular material is it has different refractive indices, so it slows down the light by different amounts, depending on what the polarization of light is.

So one polarization of the light gets slow down more than the other polarization of life.

That means that if you've got one way of coming in, which is kind of made up initially of both polarization of light, one of those waves is gonna get Bent Maur on.

One of them's gonna get Ben Tillett less, and that means that as the light travels through the images of something that you're looking at split up because the light gets shifted by different amounts in the two different polarization.

So how is that possible?

Then how come that material does this magical thing with light and your window there doesn't.

It's to do with the crystal structure of the glass.

So this is a calcium carbonate crystal calcium carbonate has his rather particular structure show you a picture of it here.

So this is actually a picture of calcium carbonate getting scarily coast chemistry here.

But basically it's made up of calcium atoms, carbon atoms and oxygen atoms in these various different colors sum total of my knowledge.

But if you look at so this is how they're actually arranged within the structure of the crystal on.

What you can see is that there's a kind of a particular access to this whole thing.

There's a ll these little little red groups kind of arranged sticking out to the sides, and then the calcium atoms are kind of in this more vertical structure, so you can see that's kind of an axis that runs up and down this particular thing in this direction here and So you will be terribly surprised if the properties of the material so remember what happens is a light wave goes through.

So imagine I've got my lightwave jiggling electric field jiggling backwards and forwards as it goes through this way.

Okay, as it goes through, it's gonna shake these atoms in this direction and shake their electrons in that direction.

Because of the structure of the very strange structure this thing, it won't be too surprising to learn that.

Actually, if you try and shake these atoms in that direction, the air react differently than if you try and shake them in that direction.

And that, at the microscopic level, is the reason why the refractive index of these two polarization of light one with the electric field jiggling one way and one with the electric field jiggling the other way are different.

And that's why the light propagates through in different way on taxes and 60 symbols.

I can't even give you a symbol for this.

It's called the buyer a fringe inside.

It's actually called Delta End, because N is the symbol that's usually used for Refractive index on what you're measuring here is a difference in refractive index.

So it's a delta for a difference.

Delta and you're seeing the light.

So really, you're seeing two images because the lights that came into this thing and kind of was reflected off that blue square, and so the blue light was reflected back to you.

One set of that blue light followed ones that appalls through that crystal on the other side of blue light followed another set of parts and they end up offset from each other.

So you see the square in different places.

I mean, the light that's that's filling This room is basically a kn polarized, but one of the things you can do is you can always split light up.

Okay, this light is kind of randomly polarized, but every little bit of it actually has a polarization at every point in space.

The light is is oscillating one particular direction on.

We could always break that an oscillation in that direction into a little bit in that direction and a little bit in that direction, so you could always break it down into two individual polarization.

Tze and that means that this complete combination oven polarized light all these different random oscillations of the light and all sorts of different directions.

We can always break down into two components.

And then it's those two components that are acting in these two different ways.

When you actually end up looking through the crystal, its excesses one is exactly wild.

That's just because you're now looking through in different directions relative to the axes of the crystal.

And so, by looking through in those different directions relative to the axis of the crystal, you're changing the refractive index of the glass.

People get go down your gem, gem and geology shop that quite often they have, um, this is a big the biggest one I've ever seen.

I just have a really little tiny one that didn't look anywhere near was impressive.