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A spectral line is the electromagnetic radiation emitted when the electron jumps from higher orbital to a lower orbital of an atom.

Water mainly consists of two elements namely hydrogen and oxygen, my question is why rainbows don't display specific spectral lines with dark gaps in-between?

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  • $\begingroup$ The optical spectra of liquids are different from the spectra of the molecules they are being made of and those are, again, different from the spectra of the atoms that form the molecules. If you wanted to see the individual atomic lines of oxygen and hydrogen starting out with liquid water droplets, then you would have to supply enough energy to first evaporate the water and then to break up the water molecules. The UV radiation of the solar spectrum does that, so one can detect these spectral lines, but they are not really part of the rainbow phenomenon. $\endgroup$ – CuriousOne May 8 '15 at 0:56
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The water droplets that create a rainbow are not emitting the light that you see in a rainbow; if they were, you would see a glowing cloud of consistent color, not a rainbow. The rainbow is formed by sunlight refracting and reflecting through water droplets in the air; the water refracts through the "front" of the drop, reflects off the "back," and refracts again on the way back out. The refractions are what separate the colors, since different wavelengths of light refract to different degrees. If you used devices capable of imaging in other wavelengths of light, you'd see further bands of "color" beyond the red and violet sides of the rainbow, resulting from the infrared/ultraviolet (and other wavelengths beyond those) radiation in the sunlight.

So in short, the full-spectrum appearance of the rainbow is due to the fact that the source of the light (the sun) is a thermal blackbody and emits a blackbody spectrum.

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    $\begingroup$ You know, I was just about to post a shorter version of this exact answer. $\endgroup$ – SuperJedi224 May 8 '15 at 0:57
  • $\begingroup$ is it correct to say that in rainbow you can observe spectral lines of the Sun? $\endgroup$ – aaaaa says reinstate Monica May 8 '15 at 2:49
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    $\begingroup$ @aandreev The spectral lines are there, of course... whether you can pick them out from the greatly overwhelming amount of continuous-spectrum blackbody radiation is a different story. $\endgroup$ – Asher May 8 '15 at 3:41
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    $\begingroup$ In principle, there might also be a bit of absorption by the water molecules in the droplets. But as we know, water is largely transparent in the visual range, so again, you could never pick this out by eye. Maybe if you could see in the infrared. $\endgroup$ – Michael Seifert May 20 '15 at 13:37
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No.

As has been said, the raindrop is not emitting the light, it is just acting as an optical device that deflects light emitted by the sun. However, the spectral lines you would expect to see in sunlight refracted by a prism will not, repeat NOT, be seen. The mechanism that produces rainbows is very different than the mechanism that produces a spectrum with a prism.

I've written a much more detailed description here: What makes a rainbow happen?. A summary is that each raindrop deflects light at all angles within a cone that has a half-width of about 40° (for spectral lines near violet) to about 42° (red). Each cone is brightest, by far, at the very edge of this cone, which is why we see colored bands. But each band we see is actually a composite of all colors from the one we perceive, to the start of ROYGBIV.

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  • $\begingroup$ when light is filtered through the raindrop due to reflective index of air and water different colors are produced due to the angle they exit, in short all visible light exits between 40° to 42° (red has higher freq hence travel faster than the rest of visible spectrums) am I mistaken? $\endgroup$ – user6760 May 20 '15 at 14:29
  • $\begingroup$ It is a very common misconception that all visible light exits between 40° to 42°, but a misconception nonetheless.Just think about it: say a ray of light is heading straight for the center of the drop. It is perpendicular to the surface of the drop when it hits, so it does not bend at all, regardless of color. It goes thru the center, reflects off the back of the drop (again perpendicular, so it reflects 180°), goes thru the center again, and exits the raindrop having exactly reversed its direction. $\endgroup$ – JeffJo May 20 '15 at 16:18
  • $\begingroup$ Therefore the rainbow I see are from reflected lights exiting the raindrops which fall within the abovementioned ranges, outside of the ranges visible light do not disperse as much did I get it this time? As for spectral lines are produced when electrons jumps between orbitals that's why there are gaps. $\endgroup$ – user6760 May 21 '15 at 0:21
  • $\begingroup$ Not sure I understand what you asked. See: ryot.org/atlantas-red-sky-double-rainbow-photos/713489. Only red light contributes to this one, so you can consider it to be a rainbow from a single spectral line. The outer rim the width of the sun; if the sun were a point, it would have no observable width. But you see red light continuously from the rim to the horizon. $\endgroup$ – JeffJo May 21 '15 at 13:44
  • $\begingroup$ Arggh - hit return for a paragraph. Each distinct spectral line that you think would be present in sunlight will have a similar disk, just smaller. They all overlap. No dark bands are possible, just dimming like in the photo. $\endgroup$ – JeffJo May 21 '15 at 13:54

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