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When a substance absorbs and reflects light, it shows the color of that absorbed and reflected light. But water is colorless. Does it absorb and reflect light. What is the reason for this?

bogus
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DARYL JOSEPH.G
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    There is a little diffference existing in the absorption of water in different visible wavelengths, thus large masses of water look blueish-greenish (even without contaminants). – peterh Apr 04 '17 at 02:46
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    See this question. – calavicci Apr 04 '17 at 03:00
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    Actually water does have a slight blue tint – Pritt Balagopal Apr 04 '17 at 06:53
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    There is also a relationship between the electrical conductivity and the absorbed light, a conductor can not be transparent (because the energy to move a electron from the valenceband to the conductionband is lower then the energy of one photon, so the photon get destroyed ) – 12431234123412341234123 Apr 04 '17 at 10:30
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    You can also ask the reverse question: why have we evolved to mostly detect only frequencies that are not absorbed by water? – biziclop Apr 04 '17 at 10:49
  • @biziclop - I mean, that's dead simple. We evolved under water, and that's simply the spectrum that is best visible there. It's completelly expected to be honest. – Davor Apr 04 '17 at 14:35
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    @Davor Of course it's a trivial observation, but it brings us closer to the answer to the original question. Water is largely transparent to the frequencies we see because our vision evolved to be that way. – biziclop Apr 04 '17 at 14:53
  • @biziclop - I figured that the question is more along the lines of "what physical properties make water transparent in the observable spectrum?" – Davor Apr 04 '17 at 15:18
  • If you only look at absorption and ignore scattering, you could boil this down to the band gap of the material making it an insulator. (yes water is an insulator) visible photons don't have the energy to promote an electron to the conduction band, whereas metals have a continuous conduction band (no gap) and therefore light can easily promote an electron. An interesting result of this is that semiconductors can have very uneven absorption characteristics across the visible spectrum (band gap energy correlates to energy of visible light photon) – Aaron Apr 04 '17 at 21:36
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    As @PrittBalagopal says, water is blue: https://www.dartmouth.edu/~etrnsfer/water.htm but heavy water isn't: http://chemistry.stackexchange.com/questions/71894/is-it-true-that-heavy-water-is-not-blue – Pablo Lozano Apr 05 '17 at 11:52
  • You may want to read this article about why water is blue. – Noctis Skytower Apr 05 '17 at 14:06

4 Answers4

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Liquid water has very little absorption in the visible light - this is why it looks colorless.

There is a very extensive article on the absorption of electromagnetic radiation by water on Wikipedia.

A few highlights:

  • there is a difference whether you are talking about solid (ice), liquid ("water") or gas (vapor). This has to do with the fact that much of the absorption is a result of molecular vibrations - and not all vibrations are possible in all states
  • Liquid water has very low absorption in the visible spectrum. Here is a plot of the absorption (note - this is for PURE water; contamination will greatly affect the transparency. For example, fine particles in water will preferentially scatter blue light):

enter image description here

Attribution: Darekk2 on Wikipedia, based on

From the Wikipedia article:

The absorption was attributed to a sequence of overtone and combination bands whose intensity decreases at each step, giving rise to an absolute minimum at 418 nm, at which wavelength the attenuation coefficient is about 0.0044 m$^{−1}$, which is an attenuation length of about 227 meters. These values correspond to pure absorption without scattering effects.

Emilio Pisanty
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Floris
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    It looks like 2+ orders of magnitude absorption between blue and red. That's a difference of attenuation length from about 1 meter to 227 meters. So even water 1 meter deep should look bluish or at least greenish. 1 meter is a lot of water except in a pool or more, or a designed narrow tube, so I can see why not obvious. – Bob Bee Apr 04 '17 at 03:23
  • In a glass of water. – Bob Bee Apr 04 '17 at 03:29
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    Can you expand on what that chart is explaining? Since the line is highest at red, it absorbs red better than other colors, which means (I'm guessing here) that it reflects other colors, which means (guessing again) it will show up blueish. – BurnsBA Apr 04 '17 at 13:11
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    @BurnsBA The chart explains that the absorption is extremely low in all wavelengths - at 700 nm it is 0.6 / m. That means that when you transmit a 700 nm beam of light through pure water, after 1 m you still have $e^{-0.6}$ of the original - about 55%. This does imply that if you filter white light through a thick column of pure water, you will lose the red components more quickly - so that the result will be a little more blue. But in real life situations, particulate scatter will have a big role. – Floris Apr 04 '17 at 13:29
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    A picture available on the Causes of Color webpage shows the slight blue color of a 3-m column of water. Look near the top of the section called "Why vibrational?" – Michael Seifert Apr 04 '17 at 13:58
  • @OrangeDog - I'm sorry, but that's what I'm saying. When you "look at" water, you are not looking at the surface - typically you are looking through the water (at the bottom of the container, etc) and conclude "no color" because the bottom looks the same color as it would if there had been no water. I didn't mean to imply that I was talking about reflection (which has much to do with the complex refractive index, and how that changes with frequency). – Floris Apr 05 '17 at 14:09
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    @Floris I @ the wrong person – OrangeDog Apr 05 '17 at 14:09
  • @BurnsBA The opposite of absorption is not reflection. Radiation can pass straight through as well. – OrangeDog Apr 05 '17 at 14:10
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Water does not absorb much light in the visible range so most visible light simply passes through. Water is, however, opaque to some other wavelengths such as microwaves.

Asher
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Our eyes are made of watery parts. If water DID strongly absorb light of some color, it would necessarily be a color that we cannot see, for which we have no name. Water vapor and many other atmospheric gasses are natural limits on ambient light, so it is not surprising that we describe them as 'transparent' to visible light.

Water (liquid and vapor) has some color in the far-infraredIR spectrum, with absorption at 3, 6, and 12 microns. So-called 'glacier ice' can be a striking blue color (possibly because of impurities like trapped air).

Whit3rd
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    Very interesting angle from which to answer this question. A sort of "anthropic principle" answer. – josh314 Apr 05 '17 at 03:36
  • If water did strongly absorb light of some color, the centimetre or so in our eyes would quite plausibly still be reasonably transparent. On another track, the colour of ice is irrelevant (the absorption spectrum of solid and liquid phases of a material can be related but do not have to be), but in any case ascribing the blue colour to air bubbles has it exactly backwards (normal ice is white due to scattering from air bubbles; glacier ice is blue because the bubbles have been squeezed out). – Emilio Pisanty Apr 05 '17 at 15:25
  • @EmilioPisanty: It's unclear that pressure would remove air; it could dissolve in the solid, instead (there's certainly SOMETHING creating color centers). – Whit3rd Apr 06 '17 at 05:16
  • In which case, if it really maters that much to you, present a solid argument and back it up with references. And, while you're there, you also need to present evidence of why it is relevant at all to the spectrum of liquid water. – Emilio Pisanty Apr 06 '17 at 06:16
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When a substance is transparent and reflects no color, at the particle level it means that the photons on which classical light rides have only elastic interactions with the medium, i.e. the atoms and molecules, and do not raise an energy level in the optical range so that the balance of perceived colors is changed.

Take a red colored crystal , a beam of light going through it turns red. At the particle level it means that there is absorption of photons from the atoms and molecules and the lattice so that the mix of frequencies coming out is perceived as red.. The same would happen if you pour red ink in water. The extra molecules of the red ink absorb the appropriate frequencies to leave the frequencies perceived as red.

Please note that the energy of the absorbed photons raises an electron to a higher level or changes the lattice position of an atom/molecule . In de -excitation there could be cascades to lower levels with a change in frequencies, but also the angular distribution of the photons coming from the de-excitation will be spherical and coherence with the beam would be lost. This means that when images are transmitted the process is elastic in order that coherence is not lost.

anna v
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