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.
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23$\begingroup$ It's not only possible, it's the case. Actually this is a well known fact. For experimental evidence, check the following link $\endgroup$– engineerMay 21, 2015 at 15:39
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2$\begingroup$ coolcosmos.ipac.caltech.edu/cosmic_classroom/… $\endgroup$– Žarko TomičićMay 21, 2015 at 16:55
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5$\begingroup$ 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. $\endgroup$– KRyanMay 21, 2015 at 19:33
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5$\begingroup$ 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. $\endgroup$– PlasmaHHMay 21, 2015 at 20:09
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2$\begingroup$ possible duplicate of Rainbows at other wavelengths $\endgroup$– Kyle KanosMay 22, 2015 at 3:27
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4 Answers
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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.
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4$\begingroup$ 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. $\endgroup$– JohnnyMay 21, 2015 at 21:57
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2$\begingroup$ Oops, Johnny is right. I edited now the images out, the link can remain. $\endgroup$ May 21, 2015 at 22:03
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11$\begingroup$ According to here, an educational forum post constitutes fair-use $\endgroup$ May 21, 2015 at 23:22
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1$\begingroup$ 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. $\endgroup$ May 21, 2015 at 23:40
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4$\begingroup$ That was exacly the reason, why I had only posted a link. Somebody else had edited the image in. $\endgroup$– engineerMay 22, 2015 at 4:24
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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.
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4$\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. $\endgroup$– ACuriousMind ♦May 22, 2015 at 12:37
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7$\begingroup$ 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. $\endgroup$ May 22, 2015 at 17:52
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$\begingroup$ -1 @ACuriousMind "If rain is made of silicon dioxide aka glass" $\endgroup$ May 22, 2015 at 22:21
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2$\begingroup$ @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. $\endgroup$ May 23, 2015 at 0:10
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$\begingroup$ @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. $\endgroup$ May 23, 2015 at 0:15
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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:
CCL image © copyright Rod Trevaskus, see geography.org.uk
answered May 21, 2015 at 18:33
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8$\begingroup$ What you see there is not ultraviolet, but a supernumerary rainbow $\endgroup$ May 21, 2015 at 19:38
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4$\begingroup$ @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. $\endgroup$ May 21, 2015 at 19:42
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7$\begingroup$ 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. $\endgroup$ May 21, 2015 at 20:17
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5$\begingroup$ @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. $\endgroup$ May 21, 2015 at 20:35
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5$\begingroup$ @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. $\endgroup$– MarkMay 21, 2015 at 21:51
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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.
answered May 22, 2015 at 0:38