How broad is the rainbow spectrum?

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    Rainbow Spectrum
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Discussion Overview

The discussion centers around the extent of the rainbow spectrum, specifically the wavelengths of light that can be refracted by water droplets in the atmosphere. Participants explore the limits of visible light, as well as the potential for infrared and ultraviolet wavelengths to be included in the spectrum observed in rainbows.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the visible spectrum of a rainbow is limited to red through violet, with potential extensions into the near infrared and ultraviolet, depending on atmospheric absorption and droplet size.
  • Others argue that while water droplets can refract visible light, it is uncertain whether they can refract longer wavelengths like radio waves or shorter wavelengths like x-rays, with droplet size being a critical factor.
  • A participant mentions that the absorption spectrum of water is narrow and suggests that the rainbow does not extend far beyond the visible spectrum due to water's opacity at other frequencies.
  • Some contributions highlight that UV light is highly absorbed by materials like glass and plastic, implying that UV wavelengths may not penetrate water droplets effectively.
  • One participant notes that the solar spectrum peaks within the visible range, suggesting a possible connection to the colors observed in rainbows.
  • Another participant emphasizes the importance of the size and shape of raindrops in determining the intensity and visibility of rainbows.
  • References to external resources and literature on the topic are shared, including graphs and mathematical details related to scattering and absorption.

Areas of Agreement / Disagreement

Participants generally agree that the visible spectrum is limited and that water droplets play a significant role in the formation of rainbows. However, there is no consensus on the extent to which infrared and ultraviolet wavelengths can be included, and multiple competing views remain regarding the specifics of light refraction by water droplets.

Contextual Notes

Limitations include the dependence on atmospheric conditions, the specific sizes of water droplets, and the unresolved nature of how different wavelengths interact with water. The discussion also reflects varying levels of understanding regarding the electromagnetic spectrum and its implications for rainbow formation.

Phobos
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We see red to violet but how much further does it actually extend? I suppose it comes down to (1) the wavelengths of light not strongly absorbed by the atmosphere and (2) the limits on a water drop sufficiently refracting the longer/shorter wavelengths. Is the invisible portion of a rainbow simply limited to the near infrared/ultraviolet?
 
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Phobos said:
We see red to violet but how much further does it actually extend? I suppose it comes down to (1) the wavelengths of light not strongly absorbed by the atmosphere and (2) the limits on a water drop sufficiently refracting the longer/shorter wavelengths. Is the invisible portion of a rainbow simply limited to the near infrared/ultraviolet?
Light is a part of the electromagnetic spectrum. The spectrum (starting from the longest wavelengths) goes: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-radiation and gamma rays. I assume there are some more on either side but I do not know of them. The wavelengths go from kilometers to nanometers.

Visible light is the only radiation that can be seen (hence the name). All of the frequencies of this portion are not absorbed by the stratosphere (well... enough can get through for us to see in all the colours of the rainbow). The visible region is limited between infrared and ultraviolet.

Also try looking at light through a defraction grating. It will show you all the colours that visible light produces.

Hope that helps a little.

The Bob (2004 ©)
 
The Bob,

I don't believe Phobos was looking for an explanation of the electromagnetic spectrum (though you did a superb job) -- I believe he was asking about the specifics of water droplets suspended in air. We all know that water droplets can refract visible light (that's a rainbow), but can they also refract radio waves? What about x-rays?

This, I don't know either. The size of the droplets is probably the most important factor. (Consult hyperphysics for some illustrations of light paths in water droplets: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/rbowpath.html) You'd need rather large drops of water to appreciably refract microwaves (radar, anyone?), and you'd need unbelievably tiny droplets to appreciably refract x-rays; such droplets have such a large surface-area to volume ratio that they probably aren't stable in air -- they'd evaporate immediately.

- Warren
 
chroot - yep, that's what I meant (but thanks, The Bob!)

Your assessment makes sense. Probably safe to rule out xrays or smaller and microwaves or larger. So I'd be curious to find out how far into the UV/IR it can go. I didn't see anything from a quick Google search...although it seems that IR was first discovered by the same curiosity of looking just beyond the visible spectrum
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/discovery.html :)
 
Yes, but Herschel discovered IR with a prism, not a water droplet. :smile:

- Warren
 
There is a really nice graph of the absorption spectrum of water in Jackson (Classical Electrodynamics, Fig. 7.9, p. 291 in 2nd edition). It shows that water is transparent only for a very narrow frequency range that coincides with our visible light. If you think about it, it makes sense that this is not an accident. It is already as opaque at 20,000 Angstroms as it as at microwave frequencies. Anyway, that means that the rainbow does not extend very far beyond visible.
 
chroot said:
I don't believe Phobos was looking for an explanation of the electromagnetic spectrum (though you did a superb job)
Oh... *feeling embarrassed*... I see. I do apologise Phobos. I didn't get the point. :frown: I didn't think you would have a problem with the electromagnetic spectrum but for some reason I thought that was what you were asking. Really quite sorry.

The Bob (2004 ©)
 
This seems like a question for ZapperZ. Can anyone get his attention?
 
Ok.. Phobos got my attention...

Not sure if I can add anymore. Krab has give a sufficient explanation for it. I do know the UV part of the spectrum is highly absorbed by something as simple as a piece of clear ordinary glass, and those cheap plastic goggles that students use in labs (and I don't mean computer labs either! <looks at neurocomp>). So it would make sense that the UV end would not get very far through a water droplet.

The IR part, I am not sure...

Told you I can't add too much to this... :)

Zz.
 
  • #10
This link has a chart showing the entire solar spectrum. I cannot help but notice that the peak of the solar spectrum corresponds with the center of what we call the visible spectrum or most of what we see in a rainbow. Coincidence, I doubt it, intelligent design... LOL.
life forms learning to use an abundant energy source.. most likely.
 
  • #11
This seems like a pretty cool site, lots of data, original sources, etc on the absorption spectrum of water.

So little to add here, except perhaps fill in some details etc:
- if there's no EM going into the raindrops, there will be no spectrum (duh!); this means (for example) that there will be no UV shortward of ozone absorption cutoff, irrespective of how transparent water is to those wavelengths
- the range of sizes (and shape) of the raindrops matters, esp wrt the intensity of the rainbows (plural)
 
  • #14
First thing that came to mind when I saw this thread was the section in Jackson (that Krab mentioned). For those that have the third edition, it's in Ch. 7, Sec 7.5 E, pg 314. There appears to be a little room on the high frequency side (in the very near UV). For instance,
[tex]\alpha _{700~nm} = \alpha _{200~nm)[/tex].

Don't know what's wrong with LaTeX...the absorption coefficients are equal at 200 and 700 nm.
 
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