Sunday, November 4, 2018

atoms - The Color White


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



"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.


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