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Uploaded:2018-05-21
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The mythical green flash at sunset isn't actually a myth! Stefan explains why it happens, and how you can see it.

Hosted by: Stefan Chin

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Sources:
https://aty.sdsu.edu/papers/Zenit/glance.html
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html
https://aty.sdsu.edu/index.html
https://aty.sdsu.edu/papers/JOSA/GF-vis.html
http://www.atoptics.co.uk/atoptics/gf1.htm

Media:
https://commons.wikimedia.org/wiki/File:Verne-Paprsek-fronti.jpg
https://commons.wikimedia.org/wiki/File:Big_green_flash.JPG
https://commons.wikimedia.org/wiki/File:Double_Green_Flash.jpg
https://commons.wikimedia.org/wiki/File:Earth%27s_atmosphere.svg
https://www.istockphoto.com/photo/light-refraction-gm468614557-34250250
https://www.istockphoto.com/photo/rainbow-gm901545308-248717570
https://www.istockphoto.com/photo/fata-morgana-gm177802588-24202206
https://commons.wikimedia.org/wiki/File:Sunset_mirage.jpg
https://www.istockphoto.com/photo/a-fiery-red-sunrise-over-the-river-gm944028404-257896432
https://www.youtube.com/watch?v=iML72Jdhiuk
[♪INTRO].

Ah, the mythical green flash during a sunset. It was first made popular by Jules Verne in an 1882 novel, and it’s still leaking into pop culture through an occasional pirate movie sequel.

Except, it’s not actually a myth. I know, it’s weird. Usually we’re debunking things people think are true, not the other way around.

But the green flash is a real optical phenomenon, and it doesn’t just happen right after the sun sets. You can see it right before sunrise, too. You can see it from any altitude, and from anywhere in the world.

But you have to have the right conditions. You might think that the Earth’s air is pretty uniform, minus the occasional puffy cloud or two. But there are a bunch of invisible layers in the atmosphere, each with slightly different temperatures and densities.

As sunlight travels from one layer to another, the light refracts, or bends, just a teeny tiny bit. It’s the same principle that makes your straw look like it’s bent below the water in your glass. But the amount of refraction depends on the wavelength — and therefore, the color — of the light.

So the different colors end up separating out from the white sunlight that enters the atmosphere. The most obvious way to see this separation is in a rainbow, although that comes from a much more dramatic change in density — from air to water droplet — so it’s a much more obvious effect. And the shorter the wavelength, the more the light is refracted.

Bluer light has a shorter wavelength, so as the sun sets, those shades will stay visible longer because they can be bent further around the horizon. It’s kind of weird to think about, but basically, the red image of the sun sets first, followed by orange, yellow, and so on. There’s a second or two delay between the last visible red sunlight and the violet.

It’s not a lot of time, but it’s enough if you know when and where to look. And it works exactly in the opposite direction, too, with sunrises. But if the blue and violet shades of the sun are also above the horizon, why are these flashes green?

Well, shorter wavelengths of light are scattered more after colliding with molecules in the air — that’s why the sky is blue. But that also means violet and blue light are more likely to be scattered away from your line of sight, so you see the sun as green. If the air is super clear, though, you can see blue flashes.

And if it’s super hazy, enough green might be scattered to make the flash look yellow, instead. But there’s a reason you don’t see a flash with every sunrise and sunset — by itself, this refraction isn’t enough for you to actually be able to see the flash with your own eyeballs. The physical separation between the different colors isn’t large enough.

So we also need a mirage to magnify the effect. Mirages are just multiple images formed by atmospheric refraction. You might think one of those images is ‘real’ and the rest are ‘fake’, but that’s not the case.

They all come from a single source. The mirage in this case looks like a second sun. You know those pictures of sunsets where it looks like the sun grows a little stand at the bottom?

That’s actually two mirage images overlapping, and it’s what allows us to see green flashes. And during sunsets, there’s also a physiological component that can amplify the effect. When you look at a reddish sunset, the receptors in your eyes that detect red light get so used to being activated that when the source goes away, everything looks more green than it really is.

But this doesn’t happen in sunrises, because there’s no red sun too look at — the green rises above the horizon first. While green flashes can theoretically be seen anywhere on Earth at any time of year, they’re best spotted above an unobscured horizon, where the air is clean and relatively stable. Which is probably why so many stories of them come from people on boats.

So if you find the right spot, you might just get a glimpse of what Jules Verne described as “the true green of Hope.” Thanks for watching this episode of SciShow! If you love science as much as Jules Verne apparently loved the green sun, well, you’ve come to the right place! And if you want a new video every day, just go to youtube.com/scishow and subscribe. [♪OUTRO].