So what we have here is the optical portion of the electromagnetic spectrum.

This is, of course, the rainbow that's familiar to everyone.

Our colors going from red to yellow to green to blue indigo, violet.

And as we progress through the rainbow, the energies associated with the photons of light um, get higher and higher and the wavelengths get smaller and smaller light is simply ripples in the electromagnetic field.

The wavelength or the size of those ripples governs the energy and the ends, the color of the resulting lights.

But what we see here in the optical regime, what our eyes were sensitive to is only a tiny fraction of the story.

In fact, the electromagnetic spectrum stretches all the way from X rays to radio covering in between the ultraviolet infrared gamma rays, all sorts of other different regimes that we may not necessarily think of as being light but certainly are and certainly are of use to us astronomers.

But by focusing in on only one portion of the spectrum.

It's as if we're limiting ourselves to borrow an analogy from another astronomer, David Helfand.

It's as if we're only hearing one section of an orchestra when listening to a symphony.

So as astronomers, we like to exploit multi wavelength astronomy.

We like to hear the whole orchestra in play.

But by listening to the various different instruments, by isolating the various different wavelengths, we can learn different things about the objects that we're looking at.

So the part of the spectrum we want to talk about today is actually the invisible bit off the end of here.

So as we go past the blue and the indigo violet, we get to the ultraviolet.

We get the wavelength that are too small for our eyes to be able to see.

Although some people with a medical condition called a fake you where they're missing a lens in there, I actually are able to see wavelengths down in this regime and see in the ultraviolet lights.

Thanks, Paul.

Um, so the son, of course, radiates at all wavelengths.

But the amount of light that gets down to us here on the ground varies according to wavelength, and that's a good thing, because although most of the light in the optical regime from the sun makes it all the way down to the ground and we can see it with our eyes, the ultraviolet light is largely blocked by the atmosphere.

And that's a good thing for us because the energetic photons in the ultraviolet are bad for our health.

They cause sunburn, they cause cancer.

And we want that layer of the atmosphere covering us and protecting us from those energetic rays.

So that's not to say that UV light is entirely bad.

In fact, we need some of it.

We need you ve photons to create vitamin D in our skin.

That's the only we don't get vitamin D from our food.

So we need sonics some sun exposure in order to be healthy.

UV Light has lots of other practical applications.

It could be used in medical diagnostics.

It can be used to monitor hygiene you may have seen in crime dramas.

UV black lights being used thio see splatters of blood or bodily fluids on crime scenes.

We can also have a lot of fun with it because there's lots of of ordinary household objects that actually glow when put under UV light.

This is called a black light.

Yeah, well, you're seeing you're seeing light coming out in the visible because the the the lamp is not perfectly emitting in the UV, but it does have some filters that blocks out most of the visible light.

But we can actually make that ultraviolet light visible using a process called fluorescence.

Lord spoke.

But so many ordinary objects undergo process called fluorescence.

They absorb this invisible light and admit it at wavelengths that we can actually see.

And you can actually have a lot of fun with this.

So historically, for a long time, physicists have known that the substance quinine is it quite under coining, I don't actually know.

For a long time, physicists have known that the substance called coining fluoresce is under UV light.

And of course, climbing is a very important ingredient in tonic water, which is very important ingredient in gin and tonics.

So I have a bottle of Kwan of time water here, so we'll stick it under the UV light.

Here we go.

I'll put the tonic water under the UV light, and if we turn off the lights, we get a terrific glow on What's happening here is that the energetic photons from the ultraviolet are being absorbed by the molecules of quinine on DDE.

Electrons in those molecules are being excited to higher energy level.

Once they're there, they start losing energy again.

First they lose it a bit through vibrational energy, and that's possibly really emitted as heat.

And so by the time they fall back to their ground state, they don't have as much energy to get rid of.

And so the energy they admit is lower energy than the energy they absorbed.

And that means the wavelength is longer.

So while they've absorbed ultraviolet lights, they emit light in the optical.

And in this case, we got this lovely bluey green glow lights.

So course you can have some fun with this because you can start making things with tonic water.

So last night I whipped up, um ah, batch of lime green jelly or Jell O for a North American viewers on.

We can see again if we put that under the black light lights.

So on the left we have jelly made with the tonic water on the right.

We have jelly made with just ordinary water, and you can see the difference.

The tonic water continues to fluoresce, even though it's in the jelly lights, all right, But Quinn is not the only substance that fluoresce under UV light that you can find in your kitchen.

In fact, olive oil is another good thing to have a look at on.

So what I've done here is I've taken some olive oil, and you can see it's quite agreeing color.

And that's because of infused it with cement, which is gonna make this effect even more spectacular.

And so you note, it's the same sort of greeny color as the jelly is, but when we put it under the black light, we'll see something quite different.

So lights.

So now you can see that instead of glowing greeny blue, it's girling, this beautiful shade of red on.

The reasons that's doing that is because of the chlorophyll in the olive oil and in the mint that I've added.

So again we're seeing the same phenomenon.

We're seeing fluorescence, energetic ultraviolet photons, we absorbed being re emitted at lower energies at longer wavelengths.

In this case, the wavelength is long enough that it comes out as red.

So I discovered that particular secret thanks to our lab group poll, who told me the story of going through a supermarket with a UV black light to find the freshest olive oil?

Because this effect doesn't last very long.

You can have even more fun and games because you can buy commercial paint and, um, skin pigments that allow you to do fun effects under the black lights again.

Got some face paints here?

If we turn the lights off, you can see the fantastic colors.

So put together these face paints and glow in the dark jelly shots, and you could have a really good party because you get marker pens that allow you to write invisible messages that can only be seen under under black light as well.

So we had some fun playing with those life, so this is all fun and games.

But to an astronomer, evey light is really important because in the universe, ultraviolet light is generated by hot young stars and other extreme objects like supermassive black holes.

So we use ultraviolet telescopes, which we have to position above the earth's atmosphere because otherwise none of the light is gonna get to us, and we use these satellites to image the universe and collect those ultraviolet photons on that can give us a really different picture of what the universe looks like in my own research.

I mostly work on clusters of Galaxies, which are made up of collections of stars that are old and red and past sort of the active phase of their life.

So there's not a lot of ultraviolet flux coming from those Galaxies.

But when I look at the same field using ultraviolet telescope like the Galax telescope, my Galaxies disappear, they become invisible and I can't see them.