Some substances like copper sulfate for example have vivid colors. But why is water transparent? Does it not emit any visual light from the electromagnetic spectrum?

  • $\begingroup$ A portion of the explanation can be found at this site. $\endgroup$ – Nicolau Saker Neto Dec 3 '14 at 21:41
  • $\begingroup$ @NicolauSakerNeto So it is because of the reflection of the light and the interactions between the photons with light. But however, I thought water was transparent. Take tap water for example. Why transparent? I understand why it is blue (because of reflections and adsorptions) but how & why is transparent? $\endgroup$ – Asker123 Dec 3 '14 at 21:45
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    $\begingroup$ did you mean "colorless"? $\endgroup$ – mykhal Oct 30 '18 at 12:25

Water is not, in fact, colourless, but is slightly blue. As you can see below, in the visible range, there is slightly more absorption toward the long wavelengths, so more blue light makes it through than redder light. However, the absorption is very low in the visible range so the blue colour is faint and not apparent unless looking through a significant thickness of water. The reason things like $\ce{Cu^2+}$ solutions appear so vividly blue is because they also absorb red light, but very strongly, so a good deal more blue light than red passes through the solution. (see here)

I'm not sure exactly what you mean by transparent here, but if you mean why isn't water opaque, because water doesn't absorb much visible light (as described above), and doesn't contain any non-uniformities (like particles or the fat globules in milk) that can cause scattering, photons can travel through water in a straight line and most make it through without being absorbed, making water colourless (when looking through a small quantity) and transparent, much as air is.

water absorption


  • $\begingroup$ As for the transparency, water is not really transparent, see: en.wikipedia.org/wiki/Deep_sea $\endgroup$ – ssavec Dec 4 '14 at 7:31
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    $\begingroup$ That was kind of my point. Transparency generally refers to the ability to pass light without scattering. If you have enough of something that it can prevent light from passing due to absorption, it's said to be opaque. If you have less and some light gets through without scattering, then it's partially transparent. Transparency is a property of a certain system, not an intrinsic properly of a material. Even metals and such can be said to be partially transparent in thin films. $\endgroup$ – Michael DM Dryden Dec 4 '14 at 16:58
  • $\begingroup$ @MichaelD.M.Dryden Thank You for clearing this up. $\endgroup$ – Asker123 Dec 4 '14 at 17:00

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