Why is Water Blue?

Lindsay — Sat, 24 Nov 2007 19:06:48 +0100

As white light travels through water it is not only refracted and scattered, it is absorbed or 'attenuated' by the water. Varying wavelengths, or colours, are absorbed at differing depths; organic matter also adding to this absoption.

By one metre below the surface, sixty percent of light will be absorbed and by ten metres eighty-five percent. The first colours to be attenuated are red, yellow and orange, which is why when one is submerged, the topography looks blue or green. In natural light, marine life that is red will appear blue at depth and only by using artificial light will the true colours be revealed.

The attached underwater image is of a sea-pool and was taken today around 2pm. The rays of light are visible as they penetrate the surface of the water; the pool however remains orange in appearance, this is due to the fact that the depth was only a quarter of a metre.

To our knowledge the intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions. Other materials owe their colors to the interaction of visible light with the electrons of the substances.

Sea Pool

Lindsay — Sat, 24 Nov 2007 19:04:19 +0100

Sea Shadow

Lindsay — Sat, 24 Nov 2007 18:11:15 +0100

If water absorbs light wavelengths at differing depths, giving the appearance of either a green or blue topography, would a shadow created at the surface still be visible at depth as long as there is light? Is a shadow in effect a Faraday cage for visible light, creating a negative space or an absence, a 'border' through which this type of electromagnetic energy cannot pass?

Tentsmuir

Why is Water Blue?

Sea Pool

Sea Shadow