Deep-water colors
 Q: Why do some colors
appear black when
they're in the water? I
know it has something
to do with wavelengths,
but I don't really
understand it.
[NOAA] Greenish-blue light penetrates from above and shines on the exploring
NOAA robot.
A: Water transmits light
of every color and,
therefore, looks clear.
The clearness enables
you to see colors of an
underwater object near to
the surface. Water is
clearest for colors of
shortest wavelengthsthe
blues and greens. That's why water looks greenish-blue and the deeper the water, the more
greenish-blue it looks. This effect is most noticeable in the water looking up. Looking down into
the water, reflections and refractions on the surface can interfere.
Water isn't perfectly clear, though, especially for colors with longer wavelengths. It strongly
absorbs infra-red (heat) radiation and weakly absorbs colors in the red and orange part of the
spectrum. White light consists of red light plus its complement, greenish-blue light. So absorbing
orange and red colors from white light leaves greenish-blue, the intrinsic color of water.
This also means that orange and red colors in deep water look black if illuminated by surface light.
Think what a "red" fish means. The fish looks red, not because it emits red light, but because it
reflects red light. If, at depth, there's no red light left, then a red fish will look unlighted or black.
Water molecules absorb red light weakly and infra-red strongly because the molecules vibrate
greatly in the infra-red frequencies. They resonatethat is, they vibrate a greater amount at that
frequency like a gong vibrates and "bongs" when hit with a hammer.
The vibrating molecules get hotter due to their motion, like those in a bowl of water warming in
the summer's sun.
Occasionally water molecules absorb visible red light. Since the effect is faint, it takes many
moleculesabout nine feet of water depth--to absorb enough red light so the blue of the water is
noticeable. That's why a glass of water looks clear.
Water is a remarkable substance: it is the only abundant material whose color is due vibrating
molecules Click to learn how they vibrate. Light interacts with electrons to color everything else
we see.
Further Surfing:
Why is water blue? By Charles Braun (Dartmouth) and Sergei Smirnov (N M State U)
Why is the sky blue? By Paul Doherty, Exploratorium Teacher Institute
NOAA photo library
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