Also interesting:
Why is milk white?
Quote:
Why is milk white? was a question asked in my Summer, 2003 newsletter from the dairy that delivers my milk, Royal Crest Dairy in Denver. The answer given was: "Milk contains casein, a milk protein rich in calcium that is white." Well, whether it is the casein or the calcium that should be white, this answer is incorrect.
The casein is soluble either in the water or the fat of the milk emulsion, and would be colorless and transparent in either case. Calcium, of course, is not white either. Even calcium carbonate is clear and colorless in a good crystal.
The whiteness is due to the scattering of light by the colloidal particles of the milk emulsion. Just the fact that we get white shows that there is no absorption, so whatever is there is transparent. This is true of most whites. The excellent white pigment titanium dioxide is actually made of clear crystals, dispersed as colloidal particles. Nearly all whites are the result of scattering from colloidal-sized particles. Colors, like green, may be produced in transmission by absorption of other colors; no such process can produce white, which in fact is a subjective color, existing completely in the visual sense. White is a very special color.
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Minor quibble, UT: White is ALL colors. If sunlight, containing all wavelengths of visible light, is shined on a surface, we'll see what's reflected. When no specific colors/wavelengths of light are absorbed, we see what has been reflected. When we see a color, it is because all wavelengths BUT that wavelength have been absorbed by the object we see of that color. When we see black, we see an object whose surface has absorbed ALL frequencies, leaving nothing, no color, black. When we see white, it's because the surface reflects ALL frequencies of light, so we see all the colors together==white.
tw: these two posts contain technical information about the scattering and absorbtion of light. Proceed with caution.