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Water appears transparent to visible light, yet most other objects are opaque. Why is that? Is there an explanation why water appears transparent?

Is water transparent at all wavelengths, or are the visible wavelengths somehow special? If it is not transparent at all wavelengths, is there some evolutionary explanation why we would expect water to have low absorption at the wavelengths that we can see with our eyes? Is there some explanation along the lines of "because we evolved in an environment where water plays (some important) role, therefore it's not surprising that our eyes are sensitive to wavelengths where water has low absorption"?

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Good question. I can see why glass is transparent (in our solar system), but I never wondered why water is transparent. –  Francisco Presencia Mar 31 at 14:57
    
See also: Why is Glass Transparent? youtube.com/watch?v=Omr0JNyDBI0 –  NeuroFuzzy Apr 2 at 1:51

4 Answers 4

up vote 11 down vote accepted

To answer this question we also need to know why some things are not transparent and why certain things, water for example, don't behave in this way.

A substance's interaction with light is all about interactions between photons and atomic/molecular electrons. Sometimes a photon is absorbed, the absorber lingers a fantasctically short while in an excited state and then a new photon is re-emitted, leaving the absorber in exactly the same state as it was before the process. Thus the absorber's momentum, energy and angular momentum are the same as before, so the new photon has the same energy, same momentum (i.e. same direction) and same angular momentum (i.e. polarisation) as before. This process we call propagation through a dielectric, and, by all the conservations I name, you can easily see that such a material will be transparent.

Sometimes, however, the fleetingly excited absorber couples its excess energy, momentum and so forth to absorbers around it. The photon may feed into molecular (i.e. covalent bond) resonances - linear, rotational and all the other microscopic degrees of mechanical freedom that a bunch of absorbers has. The photon may not get re-emitted, but instead its energy is transferred to the absorbing matter. When this happens, the material is attenuating or opaque.

So, BarsMonster's excellent graph shows us where in the spectrum water's internal mechanics tends to absorb photons for good (thus where it is opaque) and where it behaves as a dielectric, simply delaying the light through absorption and re-emission. In a short answer, it is impossible to explain the whys and wherefores of the graph as its peaks and troughs are owing to molecular resonances of very high complexity. The graph is really as good a simple summary as one is going to get.

However, there is one last piece to the water transparency (in visible light) jigsaw that I don't believe has been talked about and that is water is a liquid. This means it can't be rivven with internal cracks and flaws. Sometimes opaqueness is caused by scattering and aberration rather than the absorption I speak of above. This is why snow is not transparent, for example. For light to propagate through a medium with low enough aberration that we perceive the medium to be transparent, the medium must be optically highly homogeneous. This homogeneity generally arises only in near to perfect crystals and in liquids, the latter tend to smooth out any flaws by flow and diffusion and thus tend to be self homogenising. Inhomogeneity is a powerful block to light: the simplified models of Mie and Rayleigh scattering show this decisively.

So in summary, water is transparent at visible wavelengths because (1) molecular resonances and other mechanical absorbing phenomena don't tend to be excited in water at visible wavelenghts and (2) it is optically homogeneous, which property is greatly helped by its being a liquid.

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Water is not transparent for deepUV and infrared. From the evolutionary point of view our eye developed to see electromagnetic radiation present at earth in the past (and now) - deep UV and infrared are absorbed by water vapor and other gasses in atmosphere - so there were nothing to see at these wavelengths.

Here is a nice explanation on why some things are transparent and some are not : http://en.wikipedia.org/wiki/Transparency_and_translucency#Transparency_in_insulators

Basically water is dielectric - and majority of pure dielectrics are transparent.

Water absorption spectra

http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

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What is the relationship between being a dielectric and transparency in the visual spectrum? –  user6972 Mar 30 at 8:37
    
Is it a coincidence that light/energy from hydrogen fusion is largely transparent to hydrogen-rich H2O? Or is there some known property of hydrogen that causes this connection? –  Patrick M Mar 30 at 19:09
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@PatrickM Hydrogen fusion happens only in the core of the sun. Sun emit light just because it's surface is hot - so it's spectra is more or less continuous. –  BarsMonster Mar 30 at 20:29
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Very interesting. It had never occurred to me that most of the radiation from the fusion must be absorbed, deflected or reflected by the enormous amount of spare fuel surrounding the core, but it makes perfect sense. –  Patrick M Mar 30 at 20:32
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Aside from the links, this doesn't answer the question. Yes, we evolved to see wavelengths of light that aren't absorbed by the huge amount of water sloshing around on our planet. But you don't explain why waters absorb IR and UV rather than, say, green light. –  David Richerby Mar 30 at 22:20

The human eye contains a great deal of water, so it would be difficult to see wavelengths that are absorbed by water. The light that gets to your retina has to pass through water, so the visible wavelengths of light are to a certain degree determined by what water is transparent to.

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That does not explain why water is transparent at any wavelengths. It is a coincidence that the majority of the energy output of our Sun is at the wavelengths that water is transparent. Had water not been so transparent to the dominant sources of light, then we would have evolved light sensing structures in which the water had been mostly expunged. –  Mark Adler Mar 30 at 16:03
    
@MarkAdler Indeed. Water isn't colourless for our benefit. –  David Richerby Mar 30 at 22:00
    
To whatever extent we define color in terms of what we perceive, one might say we've evolved sensory organs that define water as colorless for our benefit. Maybe it would be better to guess that perceiving water as colored has been detrimental to the reproductive fitness of anything with odd visual organs like that. –  Nick Stauner Mar 31 at 1:32
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We don't perceive water as colorless. It's blue. –  Mark Adler Mar 31 at 1:47
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This looks like a variation of the anthropic principle. Water would still be transparent to that region of the electromagnetic spectrum if there were no humans to observe it. –  Tom W Mar 31 at 11:52

Electronic energy levels are quantized. The bigger the box the electrons can move in the smaller the energy needed for a transition. H2O only contains 3 atoms (and small ones at that) so the box is small and the transitions are in the UV. The same goes for O2, CO2.

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