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We have seen that rainbows looks so colorful as we are only able to see only the visible light. But Do they also have ultraviolet bands and infra-red bands, that we are unable to see? I know someone has already asked the same question but I am concern about the specific ultraviolet and infrared bands only rather than any other wavelength.

Shashank
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    It's not only possible, it's the case. Actually this is a well known fact. For experimental evidence, check the following link – engineer May 21 '15 at 15:39
  • there also exist appropriate diffraction gratings http://www.edmundoptics.com/optics/gratings/ – anna v May 21 '15 at 15:43
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    http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_experiment.html – Žarko Tomičić May 21 '15 at 16:55
  • @engineer that's a great source! Could you type up an answer? – pentane May 21 '15 at 17:50
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    Neil deGrasse Tyson’s Cosmos includes an episode that describes the actual discovery of ultraviolet light in the first place as being based on the fact that an invisible ultraviolet band appears beyond violet in a rainbow. – KRyan May 21 '15 at 19:33
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    This is how as kids we got tiny bits of UV light for checking banknotes. A box with water and a mirror at the right angle. – PlasmaHH May 21 '15 at 20:09
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    possible duplicate of Rainbows at other wavelengths – Kyle Kanos May 22 '15 at 03:27
  • @KyleKanos: We should close the other way around, this question has got far better answers, and is (now, at least) better phrased. – ACuriousMind May 22 '15 at 11:01
  • @ACuriousMind: I disagree on the phrasing bit (I think both are worded fine), but this question has any answers & votes b/c it hit the hnq, not because they're great, IMO. – Kyle Kanos May 22 '15 at 11:22
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    @KRyan: Wasn't that IR, not UV? – Williham Totland May 22 '15 at 13:39
  • @WillihamTotland Ah, it may have been. – KRyan May 22 '15 at 13:41
  • @PlasmaHH I'm curious, what are you talking about? You would actually use water and a mirror to check notes? – zfrisch May 22 '15 at 21:11
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    @zfrisch: A mirror at the right angle in a shallow box of water will act like a prism and project a "rainbow" onto a sheet of paper. Put a banknote into that projected rainbows uv end, and the fluorescent ink will become slightly visible. By far not as good as UV LEDs or similar, but hey, it worked and it were the early 90s ... – PlasmaHH May 22 '15 at 21:21
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    I have always wondered about this question because sometimes in a rainbow I see illusory color ghosting on either side of the rainbow and like to imagine I'm seeing outside the visible spectrum. – Michael Feb 01 '17 at 16:41

4 Answers4

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Refraction of light in water droplets, leading to the formation of rainbows, is not limited to the visible range.

Experimental evidence, compelling due to its simplicity, is shown in the following images taken by University of College London Earth Sciences professor Dominic Fortes. Check the alignment of the rainbow with respect to the trees in each of the pictures. The UV band lies to the left of the visible band, while IR is found to be shifted to the right.

The spectral limits in a rainbow can be explained more technical by looking at the refractive index dispersion of water vapor, which can e.g. be found at refractiveindex.info. The UV, visible and near IR range lie in the wavelength region between 0.2 and 2.85 µm. The change in refractive index with respect to the wavelength leads to differing refraction angles and therefore a separation of the colors, as we know it from experience. Basically, this concept could also be extended to further wavelength ranges. Although the resonance around 2.9 µm leads to higher refractive indices for longer wavelengths again. Therefore light with a wavelength of e.g. 4.3 µm would overlay with light at 0.4 µm (both with a refractive index of 1.34). Yet, this is again only half the truth. If you look at the transmittance curve (further down on the same page), you can see that wavelengths longer than 1.8 µm are absorbed by water vapor. Therefore this is the realistic long wavlength end for rainbows. I assume similar arguments could be found for the short wavelength end, but I can't find experimental data.

Vishnu
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engineer
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    Unless you've gotten explicit permission from the author, I don't think you're allowed to reproduce those images here, even with attribution, the copyright on the page says All images and text copyright their artist/author and MAY NOT be used for any purposes without the express permission of the original artist/author. – Johnny May 21 '15 at 21:57
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    Oops, Johnny is right. I edited now the images out, the link can remain. – Thorsten S. May 21 '15 at 22:03
  • @ThorstenS.: doesn't seem to have worked? I still see them. – Harry Johnston May 21 '15 at 22:54
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    According to here, an educational forum post constitutes fair-use – BlueRaja - Danny Pflughoeft May 21 '15 at 23:22
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    You may want to add something like: check the position of each image by looking at the intersection of each rainbow with the line of trees at the bottom of the pictures and you can see that the UV band lies to the left of the visible, and the IR to the right. – Selene Routley May 21 '15 at 23:40
  • @BlueRaja-DannyPflughoeft I think it's highly likely that you're right, especially from the examples you give: the crucial question is whether the author suffers damages as a result of the use (probably the opposite here). However, it would be polite for Engineer to ask: I'm sure the good prof is easy to reach by email. THe pictures can then be put back in with a "reproduced with permission of author". – Selene Routley May 21 '15 at 23:45
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    That was exacly the reason, why I had only posted a link. Somebody else had edited the image in. – engineer May 22 '15 at 04:24
  • Does anyone know of high resolution copies of the pictures? I think the outer cutoffs of the UV and IR images may be caused by absorption of light in the water, though certainly in the IR the film response may contribute. – Chris H May 22 '15 at 09:03
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    When you get a little bit into the UV, most of it gets absorbed in the ozone layer so it never reaches any altitudes where there are water droplets to reflect it. – gerrit May 22 '15 at 09:39
  • I used the image to "compress" the colours into the visible spectrum. I made UV blue, visible green and IR red. The resulting image shows a wider than usual rainbow but with yellow almost missing and a gap between blue and green. The trees and ground are a dark red colour and the sky/clouds are blue except the bottom right quadrant which is green. Unfortunately I can't post the image due to the above mentioned copyright issues! – CJ Dennis May 23 '15 at 01:32
  • s/diffraction/refraction/ – Ruslan May 24 '15 at 06:06
  • Why do you say vapor if the rainbow is formed by droplets? – Ruslan Mar 25 '18 at 21:39
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engineer already answered it completely, I only want to add that the question is completely valid even if you already know that separation of wavelength occurs.

The thing is, some materials are practically opaque or too much transparent (refractive index is equal to that of air and no separation occurs) in infrared and ultraviolet while transparent in the visible range. Water is one material with a broad range of permissible wavelengths, but e.g. glass is not. If you would throw a massive amount of glass pearls from a plane it would produce a wonderful rainbow but you could not detect an ultraviolet rainbow because glass is opaque in this wavelengths.

Thorsten S.
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    This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. – ACuriousMind May 22 '15 at 12:37
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    Actually, this does answer (part of) the question: Do rainbows have ultraviolet bands and infrared bands? -> No, not when caused by refraction through glass. It is not a critique but an addition. – Stijn de Witt May 22 '15 at 17:52
  • -1 @ACuriousMind "If rain is made of silicon dioxide aka glass" – aaaaa says reinstate Monica May 22 '15 at 22:21
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    @ACuriousMind You can and should use an answer if you can add important information to already perfect answers. Incidentally, it does answer the question by assuring that engineer's answer is correct. Comments are not intended for that because they can be deleted any time to clean up discussions. As I wrote the answer, engineer's answer was shorter and did not include the refractive index. – Thorsten S. May 23 '15 at 00:10
  • @aandreev Amusing, but there is another material which is able to produce e.g. circumzenithal arcs: ice. So the material is important if you want to have UV or IR bows. Unfortunately ice has almost the same transmission coefficients in UV/IR so no change in comparison to water. Because other phenomena like fog bows are not good examples I needed a different material like glass to illustrate the point. – Thorsten S. May 23 '15 at 00:15
  • @ThorstenS. You can imagine rain with additives that absorb vis part of spectrum, rendering pure UV rainbow. With current chemical engineering development and manufacturing safety issues that is a real possibility – aaaaa says reinstate Monica May 23 '15 at 01:23
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Is it possible that rainbows have ultraviolet bands and infra red bands and we are not able to see?

Yes, see engineer's answer. As for whether we can see them, take a look at aphakia: "Aphakic people are reported to be able to see ultraviolet wavelengths (400–300 nm) that are normally excluded by the lens. They perceive this light as whitish blue or whitish violet". Also note that the visible spectrum isn't exact. Some people can see a little further into the infra-red or ultra-violet than others. Not much, but not everybody's perception is the same. By the by, when I look at a rainbow, esepecially from the side of my eye, I seem to be able to see a yellowish tinge under the violet. It might be nothing to do with ultra-violet, but it's interesting. Maybe that deserves a new question! Anyway, you can see something like it on this picture:

enter image description here

CCL image © copyright Rod Trevaskus, see geography.org.uk

John Duffield
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    What you see there is not ultraviolet, but a supernumerary rainbow – Thorsten S. May 21 '15 at 19:38
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    @ThorstenS. The supernumerary rainbow is in the upper left, but the band of interest is below the violet in the rainbow at the bottom right. Although, it's plausible that this is an artefact of the same interference pattern process that creates supernumerary rainbows in the first place, true. – SevenSidedDie May 21 '15 at 19:42
  • I think that might be a secondary rainbow, SevenSided, see Wiki. @Thorsten S. : thanks for that, I've had a read about it. Next time I see a rainbow I'll look more carefully and try to take a picture. – John Duffield May 21 '15 at 19:53
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    The band of yellow you refer to is actually an optical illusion caused by your brain trying to reconcile the blue color of the sky next to the purple band of the rainbow. Try it - cover the rainbow in the picture above with paper, and the "yellow" band will turn blue. There does appear to be the faint beginning of a secondary rainbow band beneath the "yellow" band, which enhances the illusion, and if you move your eyes back and forth, you might actually see several smaller concentric bands of purple as the illusion becomes reinforced. – Dejay Clayton May 21 '15 at 20:17
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    @SevenSidedDie Look up the difference between a secondary rainbow which is the faint one above the normal rainbow and a supernumerary rainbow which is exactly the band of interest below the violet at the bottom right. – Thorsten S. May 21 '15 at 20:35
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    @SevenSidedDie, the upper-left rainbow is a secondary rainbow caused by light being internally reflected twice, where the primary arc is formed by light that is reflected once. The faint green and purple arcs on the lower-right are supernumerary arcs caused by interference patterns. – Mark May 21 '15 at 21:51
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    John, your post sort of makes it sound like aphakia is something I might want to have. This is in contrast with the Wikipedia page which makes it evident that I don't. Good find, by the way. – Emilio Pisanty May 21 '15 at 23:34
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    @Dejay Clayton: If I take the image and crop out just below the violet part of the rainbow, I still see the band. As for it being UV, though, not unless the camera is capturing UV. Remember that the picture is what the camera outputs, not exactly what your eyes would see in real life. – jamesqf May 22 '15 at 05:35
  • @Emilio Pisanty: my pleasure. All : if you look at the night sky with the side of your eye, stars stand out better than when you look at them directly. There's maybe something similar going on with rainbows. – John Duffield May 22 '15 at 07:18
  • @jamesqf In Photoshop, I digitally removed the rainbow up to the purple band, and then filled in the background with various sampled "yellow" colors that were near the purple band. All of them turned out to be shades of blue, with very little yellow component. – Dejay Clayton May 22 '15 at 13:04
  • @DejayClayton But they are lighter than the blue of the rest of the sky next to it. Remember, the sky's still there behind the rainbow (even the visual center of the main yellow band looks significantly green in isolation). – Random832 May 22 '15 at 23:25
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There are three factors that need to be considered across all wavelengths: (1) the ability of the water droplet to refract and disperse the incoming light, (2) the ability of the eye to sense the wavelength, and (3) the ability of air to transmit it.

The visible range we 'see' in a rainbow with our eyes satisfies all three. UV , depending on how short the wavelength is may only satisfy the droplet's refraction and the air's ability to transmit - it may be there, but we just can't sense it with our eyes.

docscience
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