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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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  • $\begingroup$ Good question. I can see why glass is transparent (in our solar system), but I never wondered why water is transparent. $\endgroup$
    – Francisco Presencia
    Commented Mar 31, 2014 at 14:57
  • $\begingroup$ See also: Why is Glass Transparent? youtube.com/watch?v=Omr0JNyDBI0 $\endgroup$
    – user12029
    Commented Apr 2, 2014 at 1:51
  • $\begingroup$ Answer: we don't know why water is transparent. This is not surprising, because physics never provides answers to pure "why" questions. A question like "Daddy, why are there people?" cannot be answered by physics or (so far) by any known kind of science. Physics instead answers "what will happen if..." questions, which in retrospect become "what happened when..." questions. The latter kind of "what" question often suffice as an answer to a "how" question. $\endgroup$
    – Ralph Dratman
    Commented Mar 25, 2016 at 19:27
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    $\begingroup$ @RalphDratman Reinterpret the question as "using a model, show that visible light passes through water." I think you could spend all day pasting this comment on almost every question on physics stack exchange... or you could be a little more generous in your interpretation of questions. Assume that "why is this the case" actually means "show using well-established models that this happens." You know what the asker means, and you're enforcing an extremely narrow definition of the word "why" in order to intentionally misinterpret the original asker. $\endgroup$
    – AXensen
    Commented May 18, 2023 at 19:27

5 Answers 5

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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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    $\begingroup$ What is the relationship between being a dielectric and transparency in the visual spectrum? $\endgroup$
    – user6972
    Commented Mar 30, 2014 at 8:37
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    $\begingroup$ @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. $\endgroup$
    – BarsMonster
    Commented Mar 30, 2014 at 20:29
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    $\begingroup$ 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. $\endgroup$
    – Patrick M
    Commented Mar 30, 2014 at 20:32
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    $\begingroup$ 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. $\endgroup$
    – David Richerby
    Commented Mar 30, 2014 at 22:20
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    $\begingroup$ This diagram also shows why water is usually blue/green :) $\endgroup$
    – Cruncher
    Commented Mar 31, 2014 at 14:59
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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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    $\begingroup$ Just want to mention that while snow isn't "transparent", it does let a -lot- of light through, which is part of the reason that grass will die if you leave a tarp on it, but can survive the winter with a layer of snow on it the whole time. $\endgroup$
    – Kzqai
    Commented Jul 16, 2015 at 0:40
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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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    $\begingroup$ 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. $\endgroup$
    – Mark Adler
    Commented Mar 30, 2014 at 16:03
  • $\begingroup$ @MarkAdler Indeed. Water isn't colourless for our benefit. $\endgroup$
    – David Richerby
    Commented Mar 30, 2014 at 22:00
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    $\begingroup$ We don't perceive water as colorless. It's blue. $\endgroup$
    – Mark Adler
    Commented Mar 31, 2014 at 1:47
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    $\begingroup$ 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. $\endgroup$
    – Tom W
    Commented Mar 31, 2014 at 11:52
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    $\begingroup$ @TomW what Eli and Nick wanted to say was: First, there was the water, and then the fish evolved an eye that could see under water. So, if water would be transparent to IR spectrum only, our eyes would see IR spectrum only. $\endgroup$
    – Alexander
    Commented Jul 16, 2015 at 12:12
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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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Pure water is transparent. Objects that are not transparent either scatter light, due to difference in refractive index between air and the substance or they absorb all the photons at the wavelength you are observing. Skin for example is opaque largely due to scattering of visible light.

In fact liquid water is blue if you look through enough of it, such as a metre or more. You can do this easily as an experiment. Molecules absorb energy in the microwave, infra-red and visible and ultra-violet parts of the spectrum. The exact wavelengths vary for every type of molecule as molecules have different shapes, bond lengths and angles and thus the quantised energy levels have different values. Furthermore, symmetry restricts certain transitions between energy levels.

Water thus has rotational energy levels (microwave region) , vibrational energy levels (infra-red wavelength) and ultra-violet transitions. Water appears to be colourless and transparent in the visible, but if you look through a long pat of it it appears to be blue. This is because of weak overtone absorptions of infrared transitions that absorb in the red part of the spectrum, the remaining light transmitted light is blue. If you tried to look through a mile or more of pure water i'm sure that it would absorb all the photons and so appear black.

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