Snow White

Part of the holiday imagery that is anticipated each year is snow: I’m dreaming of a white Christmas and such.  And why you might ask, is snow white?  And where does the white go when it melts?   The answer requires an understanding of light.

Photons, or light particles, come in a rainbow of colors. The photons on the violet-to-blue end of the color spectrum cruise the universe in short wavelengths, while the redder ones travel in longer waves.

When photons encounter an object, several things can happen: They may bounce back (reflection), bounce sideways (scatter), or pass right through (transmission). Or they may be stopped by the object, giving up their energy and dying (absorption).  An apple looks red because most of the colors in the light spectrum are absorbed, leaving only red photons to bounce back for us to see.

Snow is white because a beam of sunlight entering a snow bank is reflected by a million ice crystals and air pockets and most of it comes zinging right back out. No one wavelength is absorbed or reflected, so snow is essentially the color of the sunlight reflecting off it, white.

And where does the white go when snow melts? Water reflects very little light, with most of it passing on through, and so the water appears clear.  So the whiteness is lost due to a difference in albedo, a fancy word that refers to the percentage of light an object reflects.  The albedo of water is low, while the albedo of snow is high.

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