The Color White

Question

How can something that is white reflect all colors? It would have to reflect all frequencies or at least blue green and red, correct? How can a two element compound, such as NaCl reflect all of these colors?

Thank you.

Answer 1

"It would have to reflect all frequencies or at least blue green and red, correct?"

Yes. It reflects all colors equally. Or, at least equal enough so that our eyes perceive the color to be white. Our eye has different sensitivity to different wavelengths, so for something to be perfectly white, the relative intensities of each color must match our eye.

"How can a two element compound, such as NaCl reflect all of these colors?"

The phenomena of "reflectivity" is quite complicated. I assume in your example you are talking about table salt, since that looks white (generally). The salt is not just made up of a bunch of NaCl atoms - these atoms are bonded together into molecules, which are then bonded together to form salt crystals. It's these crystals which reflect all wavelengths equally to appear white. The details are complicated, but essentially the electromagnetic interaction between the electrons in the salt crystal with the radiation field ("light") is such that all wavelengths are reflected equally.

Note that this is different than what happens if you shine a light on atomic NaCl (each NaCl atom on its own). Then you get atomic absorption and transmission lines.

Answer 2

Colors are named by the human perception, which goes through the retina of the eye and its receptors and is classified in the brain.

In color perception frequency raises the receptors of the colors as the rainbow spectrum we observe, and thus a one to one correspondence can be assigned between perceived color and a frequency. A frequency in the optical range raises the given color perception.

BUT , a perceived color may not have a unique frequency, and this has been extensively studied. Last century E.H.Land the inventor of the polaroid camera, showed experimentally

that the sensations we call color could be produced by combining images photographed under any two different frequencies of light, as long as they were a little bit different from each other and did not come from the farthest blue/violet end of the spectrum.

This fact is hard to find on the internet but it was impressive at the time it appeared.

So in addition to the explanation of the frequency behavior of crystals, as explained in the answer by levitopher , the particular perception of white may be much more complicated than the addition of frequencies of blue green and red, which you will find out if your read the color perception link above.