Monday, May 31, 2010

Roses are red, Violets are blue


Because all colors other than red are preferentially absorbed inside the petal. The mixture of light waves strikes the rose. The waves are bounced around helter-skelter below the petal's surface. As with a wave in a lake, after every bounce the wave is weaker. But blue and yellow waves are absorbed at each reflection more than red waves. The net result after many interior bounces is that more red light is reflected back than light of any color, and it is for this reason that we perceive the beauty of red rose.

In blue or velvet flowers exactly the same thing happens, except now red and yellow light is preferentially absorbed after multiple interior bounces adn blue and violet light is preferentially reflected.

There's a particular organic pigment responsible for the absorption of light in such flowers as roses and violets - flowers so strikingly colored that they're named after their hues. It's called anthocyanin. Remarkably, a typical anthocyanin is red when placed in acid, blue in alkali, and violet in water. Thus, roses are red because they contain anthocyanin and are slightly acidic; violets are blue because they contain anthocyanin and are slightly alkaline.

Blue pigments are hard to come by in Nature. The rarity of blue rocks or blue sands on Earth and other worlds is an illustration of this fact. Blue pigments have to be fairly complicated; the anthocyanins are composed of 20 atoms, each heavier than hydrogen, arranged in a particular pattern.

Les bleuets sont bleus, les roses sont roses,
Les bleuets sont bleus, j'aime mes amours.
(Les Misérables, Fantine, Book Seven, Chapter Six)






Source: Billions and Billions - Carl Sagan

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