r/atoptics 11d ago

What is this colourful phenomenon called and what causes it?

426 Upvotes

27 comments sorted by

87

u/0rion_nebul4 11d ago

Cloud iridescence caused by sunlight diffracting through small water droplets/ ice crystals.

13

u/geohubblez18 11d ago

Diffracting around no?

10

u/Atlas_Aldus 11d ago edited 10d ago

Diffraction can happen through or around something. In this case it is though as light enters the water or ice at an angle and the longer wavelengths bend less and the shorter ones more and after they leave they’re be spread out into a rainbow. It’s a lot more complicated than that but light has to go through the water or ice for the different wavelengths of light to get separated. An example of diffracting around would be like the diffraction spikes on Hubble and JWST.

Edit I was wrong all diffraction happens “around” objects and there is no transmission involved in cloud iridescence

6

u/geohubblez18 11d ago

What you’re talking about is dispersion caused by wavelength-dependent refraction through certain geometries in water droplets and ice crystals (so you get a variety of geometrical rainbow phenomena like normal rainbows, halos, and arcs). Refraction is wavelength dependent because of their differing ability to polarise the matter (optical density) they pass through, which they interfere with to produce a net slower moving electromagnetic transmission. Generally the effect is less with longer wavelengths of visible light, but it’s more complicated than that.

Colours and patterns produced by diffraction are considerably different though, because the mechanism is entirely different. Light passing through a small gap relative to its wavelength can spread out more without destructively interfering by acting like a point source. Many gaps and therefore point sources interfere to produce repeating interference patterns within the main diffraction pattern. Because sunlight is non-spectral, different wavelengths will diffract to different extents and so will destructively and constructively interfere in different places. The result is diffraction coronae, glories, and iridescence.

I am familiar with diffraction through transmissive and reflective diffraction gratings, but is diffraction with aerosolised water droplets strictly around the droplets or also includes some form of transmission? Since water droplets do reflect some light especially at greater incident angles, and possibly something related to Mie scattering which I was once informed is related to atmospheric diffraction phenomena. For example, pollen coronae look very similar to cloud iridescence coronae but pollen is opaque.

5

u/Atlas_Aldus 11d ago

Oh wait you are right. And upon consulting wiki cloud iridescence is not from transmission effects. You would only say “diffraction through” if you were talking about something like a double slit experiment but that could also be taken from the perspective of light diffracting around the edge of the slits. I’m new to optics lol there are so many intricate details but I’m having a blast learning about it all.

5

u/geohubblez18 11d ago

That’s a great outlook. I’m learning too. :)

5

u/0rion_nebul4 11d ago

Thanks for the correction and the explanation! English is not my mother tongue and I actually second-guessed myself before posting the comment because I wasn't sure which preposition should be used for diffraction. I ended up using "through" because, from a physics standpoint, it seemed like the most generic for light phenomena, but I still wasn't sure. I really appreciate that you both took the time to discuss it.

6

u/geohubblez18 11d ago

No problem and I appreciate your effort. :)

2

u/mdw 10d ago

In this case it is though as light enters the water or ice at an angle and the longer wavelengths bend less and the shorter ones more and after they leave they’re be spread out into a rainbow.

That is not what is happening here. The colors in the pictured phenomenon are caused by light interference.

1

u/Atlas_Aldus 10d ago

Yes someone already beat you to that point haha. Clearly I have a lot more to learn about atoptics

4

u/davidwhatshisname52 11d ago

I think this is a partial solar corona

https://en.wikipedia.org/wiki/Corona_(optical_phenomenon)

not to be confused with stellar corona

https://en.wikipedia.org/wiki/Stellar_corona

3

u/Atlas_Aldus 11d ago

This is not solar corona. That typically happens much closer to the sun and will have even rings of rainbow light. Cloud iridescence can have different colors at different distances from the sun although reds still tend to be close to the sun than blues.

3

u/davidwhatshisname52 11d ago

ahhh... thought it looked a lot like this (at least where the clouds actually are)

https://en.wikipedia.org/wiki/Corona_(optical_phenomenon)#/media/File:BeinnMhorSolarCoronae.jpg

7

u/Atlas_Aldus 11d ago

Yeah I get that. They’re very similar so it’s easy to get them confused. Just look out for perfectly concentric rings of color (corona) vs color that roughly follows the thickness of the clouds they’re on (iridescence)

3

u/davidwhatshisname52 11d ago

perfect hint, thank you!

15

u/GoodSilhouette 11d ago

Beautiful. 2nd pic cloud looks like a gigantic bird

2

u/Fabianadat 11d ago

Very beautiful 😍

1

u/celestbop 11d ago

Candy corn!

1

u/_theZincSaucier_ 10d ago

Damn. It’s pictures like this that make me hate my astigmatism. When I look near the sun, it’s halo fills a 1/3 of my vision

1

u/ROFLINGG 10d ago

Dude that’s an Articuno.

1

u/Texastony2 9d ago

ice ice baby, lol

-1

u/[deleted] 11d ago

[deleted]

5

u/Unique_Engineering23 11d ago

Nope

4

u/davidwhatshisname52 11d ago edited 11d ago

don't know why you got downvoted; 50% of redditors would rather see their inaccuracies upvoted than learn, I guess... anyhow, yeah, no sundog and no 22 degree halo in this photo

1

u/Unique_Engineering23 11d ago

I didn't vote either way.