The World in UV

Hey, you look purple! I guess I should come clean. Can you smile for me? Eating my two front teeth are fake. Oh my god, they're purple! And fake teeth look different than real teeth in the ultraviolet. That's crazy!

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At first glance, the world through a UV camera looks like a black-and-white version of the normal visible light world. I mean, these white flowers are white in UV. This black towel is still black in UV, and this makes a lot of sense because ultraviolet light is very similar to visible light. I mean, it's right beside it on the electromagnetic spectrum. The wavelengths are just a little shorter, the energy per photon a little higher. So I would expect, as my first order assumption, that all matter should interact roughly the same way with ultraviolet light as it does with visible light.

But once you take a closer look, you find that the world in UV is so much stranger and more wonderful than you could have imagined. For example, it looks a lot hazier. Visibility is reduced, and clouds kind of blend in with the sky. It's like there's this fog everywhere. So why is this? I want you to try to work it out before I explain it at the end of this video.

Now take a look at these glasses. In the visible part of the spectrum, they are transparent. They look like regular lenses, but in the UV, they are almost black. They're absorbing so much ultraviolet light. Now take a look at this filter. In the visible, it looks black, but now in the ultraviolet, it looks transparent or at least translucent. This is a UV pass filter; it's absorbing all the visible light and allowing the UV to pass through it. These two objects flip my intuitions about the world on their head.

Then you come to something like this. In one of these bottles is soda water, and in the other is tonic water. Can you tell which is which? It's pretty hard to tell just using visible light, but in the ultraviolet, it's obvious. This one looks like Cola. So why is that? Well, it's because [Laughter] there are some molecules in here that interact with ultraviolet light differently than ordinary water. To really see what's going on, we've got to take this inside.

Same bottles, ultraviolet lamp. When I turn it on, it's the tonic water that is bright, whereas outside it was dark. How does this make any sense? Well, the thing is tonic water contains the quinine molecule, which was originally a preventive medicine against malaria. But it tasted so bitter that people mixed it with sugar water and made a tonic. Now, some people quite liked that tonic, but the thing about the quinine molecule is that it fluoresces, meaning that it absorbs ultraviolet light and reradiates it as visible light. That's why in here it looks like it's glowing, but outside when we're looking at it in the ultraviolet, it looks dark because it is absorbing that ultraviolet energy.

There are some other products that have fluorescent molecules in them, often laundry detergents. So I'm gonna try this one out. Does it look any different? They basically look the same. One of the ideas with laundry detergent is to make things look really bright, really white by absorbing wavelengths that you can't see, like ultraviolet, and then reradiating them in the visible part of the spectrum, which in fact makes them look much darker in the ultraviolet because they are absorbing that energy to reradiate it in the visible.

Now, if you're trying to find things that look different in ultraviolet light as opposed to visible light, well, flowers are a really good place to start because bees and other insects can see ultraviolet light. And so plants have a reason to evolve pigments that they can see. Looking at these sunflowers, you can see on the inner parts of the petals there's actually this really dark pigment in the ultraviolet; it's almost black, but you can't tell that at all just looking with your eyes. The way these pigments work is that they are molecules that have energy transitions which correspond to the energy of an ultraviolet photon. So a UV photon comes in, hits an electron and excites it up to a higher energy level, and then the electron can actually die-excite in these molecules by transferring its energy to the molecule, to the bonds in that molecule, causing them to bend and vibrate and stretch.

So effectively, it's transferring the UV energy into thermal energy and ultimately to heat. Our skin has molecules that do the same thing, specifically melanin. And I'm bringing in Diana, the physics girl, who has been filming this video with me because she knows a lot about melanin.

Oh yeah, because I'm so tan, right? So your body wants to protect itself against UV rays, and so there's some cells in your skin called melanocytes that produce more melanin when UV rays hit your skin. And that melanin absorbs a lot of visible light, so it looks darker in the visible, which is why when you have more melanin, you look more tan.

Exactly! Melanin actually absorbs even better in the ultraviolet part of the spectrum; that's where its peak is. And so our skin looks darker in the ultraviolet camera than it does in the regular visible camera.

Yeah, what I find so amazing is that these melanin molecules are transported inside the cell to the nucleus, forming this protective cap over the place where the DNA is stored, and that way they prevent ultraviolet light from penetrating into the nucleus and causing damage to the DNA. But you know, one thing I think was really interesting looking at your face in the UV... how do I put this? I mean, your skin is so clear and beautiful in the visible part of the spectrum.

Yeah, bring it on!

So what is surprising is how you can see a lot more contrast, maybe some freckles that I didn't even know were there.

It is surprising. No, but it's true! I mean, I noticed that on yours as well. I think there's a bit around your eyes there are spots I can't notice just looking at you in the visible spectrum, and then they pop right out when you're looking through a UV camera. And you know some animals that also absorb UV? I don’t know about harp seal pups; in the Arctic, when they're very specific it is... but when they're trying to do aerial surveys and take photographs of colonies of seals, it's pretty easy to spot the adults because they are dark-colored.

But the pups, I guess being young and defenseless, they have a kind of camouflage; their fur is white in the visible part of the spectrum, and so they kind of blend into the ice and snow. But in the ultraviolet part, they are absorbing a lot of that radiation. I think the ingenious thing about this is it allows us to get an accurate count of harp seals, right? Because we can count the pups too, by taking photos in the ultraviolet. So this is actually one of the uses of ultraviolet photography. You can also look for arctic foxes or polar bears using the same technique because they too absorb ultraviolet.

And that makes so much sense because these ultraviolet rays have high enough energy that they can do damage to your DNA and cause cancers and other diseases. So what should you do if you are in an environment with a lot of sun, as we are, but maybe you are not evolved for this kind of climate?

Yeah, I would say if you can't find some shade, put on sunscreen! It's kind of like putting melanin over the top of your skin.

Yeah, I mean, the active ingredients in sunscreen aren't melanin, but they do effectively the same thing. They take ultraviolet light and they convert it into thermal energy, into heat.

Right! Right! I mean, a lot of ingredients in sunscreens... some actually reflect ultraviolet as well, but a lot of active ingredients in sunscreen absorb ultraviolet light and change it into heat.

This is what it looks like if you don't go outside in the sun very much.

I just use sunscreen.

Is that right? Is that right?

Well, not every day, but I remember my mom was like, "You sound green." I'm like, "Okay!"

So why does the world look so hazy in the ultraviolet? My first thought was that it must be something in the atmosphere that is absorbing the ultraviolet light, like pollution or just one of the components of our atmosphere. But as physics girl pointed out to me, if things were really absorbing ultraviolet light, well, the sky would look darker, not light, not foggy like it does. And of course, if you do look at the absorption spectra of the common atmospheric molecules, you'll find that virtually nothing absorbs in the UV.

So what is going on? Well, the answer is Rayleigh scattering. The same phenomenon that makes our sky blue is responsible for the reduced visibility, this foggy hazy appearance in the UV. The shorter the wavelength of light, the more likely it is to scatter off tiny molecules in our atmosphere and scatter in all directions. So when we look up, we are actually seeing a blue haze blocking out space. It's a really cool way of thinking about the blue sky.

Rayleigh scattering is inversely proportional to the wavelength of light to the power of four, so it really strongly increases the shorter the wavelength becomes. In fact, there's about five times as much scattering of ultraviolet light than there is of visible light in the middle of the spectrum.

So you know, if we could see ultraviolet light, we might look up and ask not why is the sky blue, but why is the sky ultraviolet? And this is, I think, the amazing thing about seeing in the ultraviolet. It's that it shows us how the world is not how we perceive it to be. There's so much more going on there than we would ever have imagined.

Oh my gosh, oh, that's very interesting! You know, I want more of this. You know, it's one thing to know the way the world works, but it's another to see it.

So you're doing a video on different SPFs and all the sunscreen controversies on your channel?

Yes, I'm talking about whether SPF sunscreen looks different with a UV camera, why SPF labeling is so weird, and why the FDA has proposed rules years ago that we still haven’t figured out how to label sunscreens.

It is contra... yeah, awesome! So I'll put a link to Diana's video. You should go check it out. Also, the "How to Make Everything" guys have made sunscreen. They've made sunscreen! I haven't seen it yet, but I will put a link to their video so you can go check that out as well.

Why is this fly only attacking me?

'Cause I'm wearing sunscreen; it likes the smelly!