|MadSci Network: Physics|
Question: Does pure water have a color?
Answer: Maybe; it depends on who you ask!
Well respected reference sources such as The Handbook of Chemistry and Physics, and Encyclopaedia Britannica refer to water as colorless. These sources are almost certainly describing pure water. Of course, even for somewhat impure water (salty, for example), it can be pretty obviously colorless in small amounts.
But what about large amounts of water? If lakes and oceans and even swimming pools seem blue or blue-ish, then might we think that water has some slight amount of color, after all?
For comparison, consider an average pane of window glass. It too seems quite colorless -- but if you have ever seen the edge of such a pane of glass, and noticed a distinct green-ness, wouldn't you then say that perhaps the glass is not perfectly colorless? In this case any apparent colorlessness of the glass derives from light only travelling a little ways through it, from one flat face to the other. There just isn't enough glass to affect the light very much. Only when light travels through a fair amount of the glass, to reach an edge, does it become sigificantly affected.
Some general background facts on colors may now be appropriate. It is widely known that ordinary "white" light consists of an even mixture of all colors, while ordinary "black" objects absorb all those colors. Any other thing, if it possesses color, has it because it always absorbs at least one of the different colors of white light. Sometimes an object absorbs several different colors, and not uncommonly it may absorb all but one. Furthermore, an object may only partially absorb one or several colors. A grey object reflects all colors at equal intensity -- all the colors in an impinging white light are partially absorbed, equally!
The four main ways that an object can interact with light is either reflect it, transmit it, absorb it, or emit it. This rule applies to each individual color of light. An object that is translucent does at least three at the same time; there is no reason we couldn't find a translucent object that reflects red light, transmits blue light, absorbs ultraviolet light, and emits green light, for example.
All the light we see gets to us via emission, reflection, or transmission. Obviously, any light absorbed on the way to us fails to reach us, by definition. Yet upon reaching us, it is THEN absorbed by the retinas in our eyes. Why are the pupils of all eyes black? Because light goes in and mostly doesn't come out; it is being absorbed by the retinas. Yet the absorption is not perfect; any eyes you've ever seen glowing at night are working like magnifiers (the lenses gather light for the retina), and reflectors (because the retinas DON't absorb all light). In daylight this effect is hard to see, but flash-cameras regularly record eyes reflecting red, much to the dismay of photographers everywhere.
End of digression. Now what about pure water? Is there anyplace where we can look at a large amount of pure water, just so we can see if it is colorless, or has color? YES! In Japan there is a large scientific tool known as the Super Kamiokande Neutrino Detector. Its main component is many thousands of gallons of ultra-pure water. And I just happen to have here a Web address of a photograph of it:
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