`` NEWTON Spectral Absorption
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Name: Suren
Status: student
Grade: 9-12
Country: Australia
Date: Spring 2014

I have found that, water absorbs different colours (wavelengths) of the visible light spectrum as the light penetrates deeper into the surface of the water. What causes this phenomenon? Does it have anything to do with the absolute refractive index of water, the refraction of light or the diffusion of water?

It is just a manifestation of the fact that even if the absorptivity of the substance (water) is fairly low at a given wavelength, as long as it is not zero, a long path will lead to a substantial absorption. Water actually absorbs strongly in the infrared, and that absorption has a slight tail into the red region of the visible spectrum. Thus, at several meters depth in water, most red light has been absorbed, and the visible light that is left is cyan in color.

Richard E. Barrans Jr., Ph.D., M.Ed.

Hi Suren,

Your observations are entirely correct. Electromagnetic absorption of water is in three basic regions. Rotational transitions are responsible for absorption in the microwave and far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur as well.

Liquid water and ice have similar spectra absorbing in the microwave region and the infrared. The infrared (heat) absorption is why the top level of water may be warm, but just several feet below, it is cold. The absorption spectrum trails off into the blue region, which is why water appears blue. Experiments prove this.

Freshwater and ocean water absorb based on Rayleigh (particle) scattering and absorption of solutes and living matter.

However, in clear eastern Australian waters, about 30K out you may prove for yourself the blue nature of water: Dive 10 meters, clear water and the Sun is East (early morning), tap your cylinder with your knife so the plankton and fish scatter - you will see the blue for a few seconds. While you are down there, enjoy commiserating with the fish.

Thank you for your fascinating question! Peter E. Hughes, Ph.D. Milford, NH

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