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Clear water is clear because of an intricate combination of physics, chemistry and biology!


At some level, this is just a coincidence.

Every material absorbs different wavelengths of electromagnetic radiation to different degrees. The absorption spectrum of water[1] looks like this.[2]

Here is how to read this graph. For a given wavelength, the y-axis gives approximately the fraction of light that is absorbed in traveling through unit thickness of water. For a more intuitive way to look at it, the reciprocal of the number on the y-axis gives you the distance (in meters) that light of a given wavelength can travel without undergoing significant absorption.

So the graph tells us two important things

  1. Even clear water is not fully transparent. Even at its most transparent wavelength, water will absorb most of the light once the depth of water is above 50 meters or so. As a result, the deep ocean floors are always pitch dark! Organisms at such depths cannot get their energy from sun and have to resort to chemosynthesis.[3]
  2. It is only a very narrow wavelength range for which water is transparent. If you move a little to the near infrared, about a few centimeters of water is enough to absorb light. Into the ultraviolet, the thickness is as low as a few nanometers!

So what is going on?

There are different underlying atomic and molecular processes that give rise to absorption.[4] There are electronic transitions where a molecule absorbs a photon and moves to a higher excited electronic state. There are also rotational and vibrational transitions where the electronic state remains the same but the rotational/vibrational state is changed. The electronic transitions occur at higher energies than the other two.

Now, it turns out that the visible spectrum falls between the electronic and vibrational absorption bands of water. We just got lucky to be able to see in this transparency window!

But this is not a coincidence, really. Our eyes evolved[5] while we were still in water. For these light detectors to be useful, they had to be photosensitive to water-transparent regions of the spectrum. Fortunately, this range also corresponds to a region where the sun emits a significant portion of its energy. As we evolved out to land and became human, the spectral range did not change by much. Remember that our eye “optics” still consists of a lot of liquid.

PS: More of my answers related to topics of my research are aggregated here.

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