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Question:
Why do objects such as quarks, which are smaller than the wavelength of visible light, have no color? If the color of an object is simply the color of the light that is reflected off it, then why cannot extremely small objects reflect wavelengths of light that are larger than themselves? On the macro- scopic scale, small objects can "reflect" larger ones. For example, if one had very good aim, one could theoretically bounce a tennis ball off a pinhead. Why is the same thing not possible on the microscopic scale?



Replies:
In some sense micorscopic things DO have color! Take the example of a single atom. When light falls on an atom, it scatters, which can be considered a reflection. This scattering depends on the wavelength of the light, with some wavelengths scattered very strongly and others not at all, just like you would say the color of a macroscopic object is determined by which wavelengths are reflected. (This selectiveness is due to the energy level structure and quantum mechanics.) This is also true for other microscopic things such as nuclie, subatomic particles, but the smaller the object, the harder to "hit". The reason we do not really say an atom has a "color" is that color is a bulk property just like roughness, for example. But by the strict definition of which wavelengths are reflected, small things are colored. Another "law" you may be thinking of is that light cannot image details smaller than a wavelength. This IS true, although some current research is trying to find ways around this law. But that is another story!

timo p grayson



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