Polarized rainbow, what does this mean???

By Phil Plait | August 18, 2011 10:20 am

Earlier today I posted about a ginormous blob of gas 80% of the way across the Universe that’s emitting polarized light, and how that’s a dead giveaway it has galaxies embedded inside it. The fact that the light was polarized helped solved a ten-year-old mystery about what’s lighting it and other cosmic blobs like it.

Now, that’s great for something that’s 100 sextillion kilometers away, but what about here on Earth? Well, it turns out polarization works down here, too. It can make rainbows disappear!

[Set the resolution to 720p or 1080p to see it best.]

Here’s how this works. First, to quote my post from this morning:

Imagine two people standing on opposite sides of a tall picket fence. There are spaces between the pickets, maybe 5 cm wide and two meters tall. One person has a sheet of plywood to hand through to the person on the other side. If they hold the plywood horizontally, it can’t get through. Duh. But if they rotate the sheet so that it’s vertical, it passes between the fence pickets easily.

Polarized sunglasses are like that picket fence. They’re coated with a very thin sheet of molecules that naturally align themselves in rows. The manufacturers make sure those rows are aligned vertically on the lenses. Why? Because light reflecting off pools of water or other flat surfaces are polarized horizontally. That way the molecules in the sunglasses’ coating blocks most of the light from reflections, reducing glare.

And that’s what’s going on in the video, too. Rainbows are created when sunlight reflects inside water droplets, bouncing back to you. When the light enters the droplet and also when it leaves, it bends a little bit as well (like how a spoon looks bent in a glass of water). Different colors bend by different amounts, so the sunlight colors get spread out, forming an arc in the sky.

The light forming the rainbow gets polarized when it reflects off the back of the raindrop. The amount of polarization is pretty strong, as the video shows. When I hold the glasses horizontally the light gets through, but as soon as I rotate the glasses, the rainbow disappears entirely! Almost all the polarized light is blocked, and the rainbow vanishes.

But wait, there’s more!

In my video, the way the setting sun was being blocked by clouds meant we could only see the base arch of the rainbow. Weirdly, had that been a full rainbow, rotating the glasses would have made the base disappear, but not the top! And holding the glasses normally would’ve made the top disappear and not the base. Why?

It’s because of the direction of the light entering the droplets to make the rainbow. The polarization happens perpendicular to the direction the light is traveling. In other words, near the base, the light you see has been bent horizontally back toward you, and the polarization is vertical. At the top of the arch, the light has been bent vertically to get back to you, so the polarization is horizontal. If it helps, think of it as the polarization being tangential to the arc of the rainbow, like in the graphic here (which I got from Polarization.com, where there’s a great explanation of all this, including how polarized sunglasses work).

Remember, sunglasses are polarized vertically when worn normally, to block the horizontally-polarized light from reflections. So when you’re wearing glasses, the horizontally polarized light from the top of the rainbow is blocked, but the base remains. Take the glasses off, spin them 90°, and boom: no base, but the top reappears. Try it after a rainstorm!

One more thing: a lot of smart phone screens are polarized as well. When I take a picture with my phone, I sometimes rotate it to fit something wide in the field, and if I’m wearing those sunglasses the screen turns black! I have to take the glasses off to see what I’m photographing.

This effect can be used to create 3D images and movies as well… but that’s another story, for another post.

So, the next time you see a rainbow, whip out your sunglasses, turn your head to the side, and yell at the top of your lungs,

Polarized rainbow!

THAT'S what it means!!!

Image credit: Polarization.com. For those unfamiliar with the Double Rainbow mem, here’s the background, and, of course, here’s the original video. Watch that, and then watch Paul and Storm’s fantastic satire of it. You’re welcome.


Related posts:

In astronomy, a polarizing view is good
Naked rainbows
Rainbow conspiracy
Fire cloud
Do rainbow clouds foretell earthquakes?

CATEGORIZED UNDER: Cool stuff, Pretty pictures, Science

Comments (49)

Links to this Post

  1. Polarized Rainbows | August 18, 2011
  2. The Vanishing Rainbow [VIDEO] - The River 100.5 FM | August 21, 2011
  3. Poimintoja, osa II « uniVersI/O | September 22, 2011
  1. Jim Craig

    Once again, Phil gives me my daily dose of awesome.

    Adding a little science doesn’t make the rainbow any less beautiful. It adds another layer to how amazing the rainbow is.

  2. Niall

    Haha, Cheers Phil! I always wondered about polarised glasses, I use them a lot when I’m doing fieldwork to help me see better underwater when I’m out wading in the name of science. I always notice that the reflective quality of an area of water disappears and reappears when I move my head slightly.

  3. Dave.

    Nice.

    I learned something new today. I never realized that the light in a rainbow is polarized

    Thanks!

  4. WJM

    Polarized rainbow all the way! OMG!

  5. Keith Bowden

    “But wait, there’s more!”

    Channeling Mona Lisa Vito again, Phil? ;)

  6. andyo

    OMG vertical phone video syndrome! HD is a lie! Otherwise, cool. We don’t get a lot of rainbows here in LA though.

  7. Jamey

    Cool – but still doesn’t really explain why the light from the LAB is polarized – at least, not to me.

  8. Messier Tidy Upper

    I knew rainbows were natural spectroscopes splitting the sunlight into its spectra (Could we see the dark Fraunhofer lines if we magnified one enough?) but I must admit I didn’t realise they were polarised like that. Very neat trick & video. :-)

  9. Bryan

    I seem to recall that all LCD screens are polarized. The liquid crystals work by forming what amounts to a polarizing filter when current is applied. There’s s backlight and a series of colored filters sandwiched between the LCDs. So if you rotate your polarized glasses when looking at any LCD the colors go crazy or black out all together.

  10. Snake’s skin also polarizes light. They are much easier to spot basking in the sun when you’re wearing polarized glasses.

  11. QuietDesperation

    That rainbow became 20% cooler.

  12. MartyM

    Polarization is used in 3D effects. I tend to like the active glasses more than the passive kind (and I don’t like auto-3D displays at all – what a headache!). Active glasses are shuttered, and each frame of video is polarized and synchronized with the glasses so that one lens lets one orientation of light in and the other blocks it with an lcd or mechanical shutter switching back and forth at at least 60 hz and possibly up to 240 hz. To me it helps prevent eye fatigue and headaches. With passive glasses you have to mentally parse the polarized video and I think that can lead to fatigue and headaches which doesn’t tend to lend itself to an enjoyable 3D experience. Though I don’t watch much 3D content anyway.

  13. Ed

    Time to correct a misconception. The long molecules are oriented perpendicular to the direction for which they allow polarized light to pass. The reason is that electrons move relatively freely along the long molecules and therefore when the electric field vector is parallel to the long molecules the responding motion of the electrons absorbs energy from the electromagnetic wave. Unfortunately, this works against the picket fence analogy and is also counter-intuitive to many diagrams one sees online. For example, this site by Nikon has the right explanation even though the diagram suggests the direction of the molecules is picket-fence-like.

    http://www.microscopyu.com/articles/polarized/polarizedlightintro.html

  14. I knew that! A B.S. in Physics has to be good for something sometimes.

  15. Benjamin

    That’s great, but not all sunglasses are polarized. Most of them don’t even polarize light. When buying them, there’s often a small sticker on the glass itself saying they’re polarized. I even had a pair of Oakley’s and the word “polarized” was engraved on the glass itself.

  16. Joseph G

    F*****ing magnets – how do they work!?

  17. John

    Interesting article! This made me think of a youtube video showcasing a strange phenomenon where the sunlight hits a cloud creating a weird lighting effect. I wonder if this has anything to do with the polarization of the lense on the camera? The link to the video can be found below. I hope you’ll be able to reply :)

    http://www.youtube.com/watch?v=E4sY98zsBH0

  18. Shane

    I just found the new name for my band. “Polarized Rainbow” Thanks Phil

  19. I remember playing around with a polarizing filter for my camera. There were two clouds in the sky at the time. I found that depending on how I turned the filter, I could make one or the other cloud disappear.
    I haven’t figured out why, though I suspect that if I reread the OP and think about it, I will. It probably had to do with the different angles at which the light was getting to me.

  20. Joseph G

    Oops, wrong meme :D

  21. Chris

    You should have put another pair of polarizing lenses 45 degrees off between the rainbow and your sun glasses. Make it magically (ahem, scientifically) reappear.

  22. Michael

    There is a whole chapter about this in Walter Lewin’s book “For the Love of Physics”, which just came out on Audible. That was the first time I’ve heard about the polarization of rainbows, this post is excellent timing.

    I’m sure Phil had lots of chances at this, we see tons of rainbows here on the Front Range :)

  23. TMB

    @13Ed – damn, you beat me to it. :-)=

    [TMB]

  24. Logicboy

    T
    h
    a
    n
    k
    s

    P
    h
    i
    l

  25. Ed @13 is right—the molecules in polarizers are aligned in the direction that they kill. The basic E+M logic is that perfect conductors reflect 100% of the light incident upon them. The long molecules are good conductors along their length (many many atoms long, lots of electrons to zip back and forth) but poor conductors across their breadth (order 1 atom across, so electrons are basically stuck in that direction).

    Also, for an excellent lecture about rainbows, see Walter Lewin’s OCW lecture:
    http://ocw.mit.edu/courses/physics/8-03-physics-iii-vibrations-and-waves-fall-2004/video-lectures/lecture-22/

  26. anon

    Phil,

    I implore you, seriously, I beg of you, embed your vidoes WITHOUT autostart. Autostart is presumptuous, arrogant, disrespectful, annoying. It is rude.

    I come here to read, and when the time comes for me to watch, I will watch. But it should be my decision, not yours.

  27. I did this with my DLSR and a last summer with a bit higher zoom.

    http://halfastro.wordpress.com/2010/07/04/rainbows-and-polarizing-filters/

    As for 3D movies, there are a couple of different types of polarizing glasses. The older ones that are used in amusement parks and some IMAX theaters used linearly polarized filters. RealD uses two circularly polarized filters which are a different beast.

  28. Naomi

    …I’m not going to be able to get the Double Rainbow song out of my head all day, now. Thanks heaps, Phil! :(

  29. VinceRN

    Cool demo, and good explanation. I’ll be using this one to help teach my daughters. Thanks.

  30. Das Boese

    Strangely enough, it reminded me of this:

    http://www.youtube.com/v/w3qFdbUEq5s

    (Warning: extreme stupidity ahead, but it’s hilarious)

  31. Mount

    Seat-of-your-pants science, great stuff! Good work even just thinking of that at the time, and posting a relevant video.

  32. Daniel J. Andrews

    I used a similar analogy (re: picket fence) when I used to teach first year biology in the lab lesson dealing with microscopy and cross-polarization (two polarizing filters turned ‘right-angles’ to each other so the light is blocked…like two picket fences being at right angles to each other so the plywood won’t get through no matter which way you turn it). Of course now after reading Ed’s explanation I feel guilty for misleading students.

  33. Jamey

    @Daniel – actually, you taught them a *model* that is correct in gross, but not in fine detail. This is simply the way we teach *everything* these days relating to science – it’s just that as we get older, the balance between incorrect gross details, and the correct fine details, gets balanced differently. Elementary students get atoms as balls – middle school, you get the planetary system version – high school, you start to get the shell model, and how those shells aren’t really spheres – university, you’re tasked with computing the waveforms that describe the shells, and how those waveforms are modified by interactions with other particles along the way. The middle school version is wrong, sure – but it’s *less* wrong than the elementary school version. Even so, for things such as developing the gas laws, the elementary school version does pretty well for the normal range of temperatures & pressures.

    Asimov had a nice essay on “The Relativity Of Wrong” which looks at this.

    Oh, and THANK YOU MATT HAYES! for answering my question on why the light from the LABs are polarized, back in the previous post’s comments!

  34. Robin

    @ Messier (#8): Given sufficient sensor resolution or if viewing with the eye, sufficient magnification AND sufficient lens resolution, you could see the Fraunhofer lines. I’d argue that water droplets (given the correct angle incidence of light) do not split light (that’s a what beamsplitters do) but rather disperse it. After all, they produce a continuum of light with (in terms of viewing with the eye), discontinuities at the Fraunhofer lines. Besides, the correct optical term is dispersion.

  35. Robin

    @ Bryan (#9): ….but if you’re a crafty git with special resources and you happen to have a pair of glasses that are made with circular polarizers, you can rotate your head all day, and the LCD will never appear to dim. What you describe, though, can be an issue for some, like cyclists. Some unfortunate souls get sunglasses that are partially or completely cross-polarized to the light from their bike computer, making it difficult or impossible to see their computer screen without rotating their heads.

  36. Robin

    @Hale-Bopp (#30): 3D films made with circularly polarized light are much better because the angle (of rotation) of circular polarizers does not change the polarization state of the light after passing through the filter. With 3D films that use linear polarizers, the 3D effect becomes less “effective” if a viewer tilts their head since the rotational position of a linear polarizer determines what the polarization state of the emerging light is.

  37. Robin

    @ Das Boes (#33): I’ve seen that same video used to “prove” that Chemtrails are real not the wacko conspiracy theory that they really are.

  38. Pete Jackson

    Nice pictures of dogs, except that somebody kept putting their sunglasses in the way.

  39. And that’s why I can’t use my iMac with my sunglasses on!

  40. DaveN

    The “after the fold” section addressed what I immediately thought of upon reading the first section… Which was this: What about a “circular rainbow” like you see from an airplane if you’re looking out opposite to the sun at a bank of clouds (with the shadow of the airplane in the center)…! Presumably, with polarized glasses, you would see a rotationally matched portion of the circle, but not all of it… Now how about a video of that, Phil! ;-)
    Ooh, added NB: Airplane may not be needed, if you have a ladder, sprinkler, a low sun on a clear day, polarized lens(es), and perhaps a waterproof camera case :D… See the bottom of this page for some ideas… Anyone??
    http://courses.science.fau.edu/~rjordan/busters_33/answers_8b.htm

  41. Thanks Ed, I was about to say that. Phil is wrong. :)

    also:
    long -> wide (Length is time.)
    wide -> broad
    travelling -> faring (One travels by foot, stick, or oar.)

  42. Cool. One of those smack yourself on the head things – should have known that. What I really like about it is that it is one of those extremely simple but powerful demonstrations of a physical principle that any kid can do. These are the kind of things that get kids hooked on science and we really need more kids interested in science.

  43. mike

    Plate O’Shrimp! I just discovered this phenomenon for myself last weekend, looking at a full rainbow through polarized sunglasses. Thanks for the writeup!

  44. Matt B.

    I think I discovered that many years ago and forgot. I’ll have to get my polarizing plates out and use them on everything while I take a walk. I have two, so I can rotate them relative to each other.

    BTW, Phil, you can turn the sunglasses the other way, so that the arms point away from the camera. That’ll make it easier to get them close up.

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