Blue Milk?

Secrets to milk’s color are all about deflection.

| March/April 2008

Milk and snow seem to have the white market cornered. In many oral cultures, if you wanted to give something that special glow, you associated it with milk: If you rode away on a steed or charger, chased a boar, or even spent some time with a swan, it was practically guaranteed to be milk-white. Even Steven Sondheim’s nursery rhyme characters in Into the Woods look for “a cow as white as milk.”

But why does milk look white? Or perhaps a better question, when I switched to skim milk, lo these many years ago now, why did my cereal take on a bluish tinge?

In order to understand milk’s whiteness, we must first understand a little bit about colors and the way light works. Light is really a form of electromagnetic radiation (radio waves and microwaves are, too). This radiation has a wavelength that can be measured, and each color of visible light has a different wavelength.

The color we see is dependent upon the wavelengths that enter our eye. Grass appears green because it reflects green wavelengths of light, while absorbing other wavelengths. Tar is black because it absorbs all visible wavelengths. White is special: When all the wavelengths are reflected, we perceive the color as white.

When light interacts with suspended particles (such as in air or a liquid), energy can be deflected or absorbed. The size and chemical nature of the particle determine the wavelength that is deflected. This explains the blue of the sky. Small particles in the atmosphere scatter the shorter blue and violet wavelengths sufficiently to cause the sky to appear blue.

Try this experiment: Put water in a medium-size clear glass and shine a flashlight through it. The light beam is very difficult to see in the water. (You may be unable to see it at all.) Add a few drops of milk to the water and shine the flashlight through the glass. The light beam is now visible as it passes through the mixture, due to scattering of the light. Keep adding drops of milk and, eventually, the mixture will take on a distinct blue color because the milk particles are scattering the blue light. If you put a piece of paper on the other side of the glass to catch the remaining light beam, it will be orange.

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