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I recently read an article about quantum chromodynamics which suggested that quarks do not have actually colors because their dimensions are smaller than the wave length of visible light.

My question is: can objects with dimensions smaller than the wavelength of visible light have colors? !(meaning if we can see them colored)!

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    $\begingroup$ Colors in QCD are just a tag to distinguish them. There is nothing intrinsic in them color-wise. $\endgroup$ – Ignacio Vergara Kausel Oct 8 '14 at 8:03
  • $\begingroup$ This color is not the color in the everyday sense. The name here may be misleading to those who did not touch quantum field theory. If it confuses you, remember that it has nothing to do with the color seen by eyes. $\endgroup$ – Zheng Liu Oct 8 '14 at 9:57
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    $\begingroup$ @ZhengLiu (and @Ignacio), if you read carefully, he is not asking about the misleading-ness of the coloring-label for quarks. He is using this to motivate the explicitly stated question. $\endgroup$ – Chris Gerig Oct 8 '14 at 10:06
  • $\begingroup$ Well maybe i didn't state the question well. What i want to know is for example do we see a virus with a length of 50nm colored? $\endgroup$ – George Smyridis Oct 8 '14 at 10:25
  • $\begingroup$ @Chris Gerig Yes. But still, this motivation itself is misleading, and is worth pointing out, isn't it? $\endgroup$ – Zheng Liu Oct 8 '14 at 11:50
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For usual way of looking, trying to see an object with size smaller than wavelength you shine at it will give you nothing: you'll basically not notice the object at all: it can't be resolved.

But, there's a trick, where instead of using normal "shine and look from a distance" way you watch the object very closely, taking advantage of the properties of evanescent waves — it's how near-field scanning optical microscopy works.

Using such a technique, you'll definitely be able to determine the color of a small object, although this color could be very different from the color of a macroscopic collection of such objects, because electron confinement effects may appear to be important. Example of such situation would be trying to "look" at quantum dots of different sizes: quantum dots are basically nanocrystals of the same material, just having different number of atoms in them, thus having different sizes.

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Roughly speaking, the color of an object is the wavelength which hits your eyes when [typically non-coherent] light interacts with the object. This interaction includes refraction and reflection and the spectrum of incident light. But for objects that are single particles, there aren't really any refraction/reflection properties, there is just stimulated emission. The photon which is emitted from the particle will have a wavelength which is dependent on the incident light and the energy levels of the particle. So for the purpose of saying an object has a defined "color", the answer is no.

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I'm not sure the other answers are complete. Quarks could be said to have a photopic color under certain conditions.

If there is zero interaction (other than elastic scattering) with photons, then quarks could be considrered to be "white," as they have no preferential absorption. If quarks are capable of absorbing photons, they are either "gray" or have a color if that absorption is wavelength-dependent.

I know far too little about QCD to make any claims as to whether such interactions (not counting virtual photons) occur.

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