Macroscopic objects either reflect, or transmit, or absorbs electromagnetic waves. Assume now we have, say, a red apple exposed to the usual white light. The red light is apparently reflected from this apple.

Next, the apple consists of molecules which have some complicated spectrum (from the quantum point of view). I was told two exactly opposite opinions about the relation between this spectrum and the red color we see:

1) The red light corresponds to the spectrum. Namely "red" photons first excite energy levels of molecules of the apple and then they are re-emitted again, and this is what we see. On the other hand all other colors cannot excite energy levels of molecules, and they are just somehow (how?) absorbed and eventually increase temperature of the apple.

2) All non-red lights corresponds to the spectrum, and the red light is the only one which does not correspond to the visible spectrum of molecules. In this approach "non-red" photons are absorbed exciting quantum levels of molecules and then this energy is somehow redistributed to increase temperature of the apple. The red photons are not absorbed and are somehow (how?) reflected.

Obviously at least one of these explanations must be completely wrong. Which one? What is the correct mechanism of visible colors on the molecular level?


1 Answer 1


The wrong explanation is (1). The non-red light is absorbed, electronically exciting the material and rather than re-emitting the light (fluorescence) there would be mostly nonradiative relaxation. The red light is reflected and observed. This article describes some nonradiative relaxation pathways.

  • $\begingroup$ To make sure: you mean (1) is wrong? $\endgroup$
    – MKO
    May 15, 2014 at 14:50
  • $\begingroup$ Yes. I edited to make that clearer. $\endgroup$
    – doetoe
    Apr 11, 2018 at 16:28

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