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.
The general ‘color’ question has been answered pretty well, methinks. The general question about reflection is also interesting, though … The reason the ‘small objects can’t reflect large ones’ is true at the quantum scale is because the wave aspect of the objects is apparent. When a wavefront strikes an object that is much smaller than the wavelength, it simply ‘breaks’ around it; if it strikes an object much larger, it is reflected back. A tennis ball has a wavelength in the femtometer range, IIRC-a pinhead is much, much too large for it to break around. In the subatomic range of objects, the wave aspects of matter become much more relevant. Only an incredibly high-energy gamma ray will fail to pass right through a bare neutron-and more likely it will be absorbed by & annihilate the material in the area than be reflected so you can detect it. Hence, the use of electron microscopes …
Good responses, folks. To clarify, there are two processes that cause the wavelengths (colors) of light falling on an object to be changed: scattering, and absorption. For single atoms/molecules (it makes no difference which, contrary to the FNAL web page cited above) it works like this:
Scattering: Photons (particles or quanta of light) falling on an atom are deflected by the electrons orbiting the atom. The scattered light that you “see” (i.e. with a detector of some sort, your eyes aren’t sensitive enough to see small numbers of photons) has a color that depends on two things: the color of the incoming light, and the angle that you are observing at. So the apparent color of the atom depends on where you stand (with respect to the direction of the incoming light)!
Absorption: As described by mnemosyne, atoms/molecules can absorb light of particular wavelengths. This can then be re-emitted, either at the same wavelength, or, if intermediate energy levels are available, at a combination of longer wavelengths. (BTW longer wavelengths are redder, shorter ones are bluer, if we are talking about visible light.) The result, as the FNAL site describes pretty well, is that light passing through a cloud of gas will have certain colors removed from it. For instance, if the atom absorbs yellow light, the transmitted light will have yellow removed and so will look (green??? - insert the complementary color here; i forget my color wheel). On the other hand, if you view the cloud of atoms from the side, you will see primarily the yellow that is being re-emitted in all directions (or a combination of reds, if the two-step emission process is available), together with scattered light as above.
This is the description from the physics point of view. The actual appearance depends on how the brain interprets the signals from neurons that respond in weird ways to different wavelengths of light, blah, blah… Of course that’s not the really interesting part.
