I understand that light (color) is part of the electromagnetic spectrum, and that it depends on what wavelengths are reflected/absorbed. Though what property of an individual atom gives it its color? Do electrons, neutrons, and other subatomic particles have color?

Basically: What intrinsic property causes the differences between how the varying wavelengths of light are reflected at the atomic scale? Also, how do photons factor into this?


Basically: What intrinsic property causes the differences between how the varying wavelengths of light are reflected at the atomic scale? Also, how do photons factor into this?

These are absorption lines in the solar spectrum


Fraunhofer lines coincide with characteristic emission lines identified in the spectra of heated elements.6 It was correctly deduced that dark lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere.

Emission lines of Iron are shown below.

If you have some physics background you will know that both these spectra depend on photon emission and absorption by the electronic states around atoms and molucules.

iron emission

As there are many elements in the sun part of the observed continuity of the spectrum is due to the overlap of frequencies and the multiple possible states for each atom and molecule to absorb/emit photons.

There exists also continuous emission of photons when electrons ( charged particles) are accelerated or decelerated in external spill over electric and magnetic fields that exist around all atoms and molecules. This spectrum, called black body, will be continuous, and is what one sees in an incandescent lamp or very hot iron; photons are emitted continuously by all bodies , even though not in the visible spectrum.

So the role of photons is crucial to all electromagnetic radiation, including that from our sun. As an ensemble they form the classical light wave, the creation process is a quantum mechanical one involving photons.

Reflection is again an interaction of the individual photons of the beam with the material they impact. The frequencies that are not absorbed will define the color of the material that scattered the beam. So photons are crucial to defining the colors we see.

The photons are coherently scattered by the field of the electrons of the atoms and molecules of the scatterer. The classical view of an electromagnetic wave can be shown to coincide with the quantum mechanical one and is much easier in calculating the behavior of beams, which are huge ensembles of coherent photons.

  • $\begingroup$ Okay, your wikipedia links and explanation helped me find even more webpages, and from those, more questions. What determines which energy band an electron falls to when emitting a photon? Why only emit enough energy for the x color instead of enough for color y? How do the number of electrons an atom has play a role in the re-emitting of photons? Quantity/energy differences? It seems like it should, but would be silly if it does. For black objects, where do all the photons go? Heat? They only absorb photons? How do iridescent substances or mirrors function in terms of energy bands, etc? $\endgroup$ – keroro Oct 16 '13 at 5:42
  • $\begingroup$ Sorry for the block of text, the comments don't like paragraphs... $\endgroup$ – keroro Oct 16 '13 at 5:44
  • $\begingroup$ Black absorbs almost all visible frequencies photons, yes, heat is the end result. Each element and material differs from the others by the combinatorics : atoms by number of electrons and protons, molecules by number of external possible levels not filled, solids by total structure (as in crystals and glass) etc. An electron has to fall back into an empty energy level ( of the total material, it is only in gases we have individual atoms free from interactions with others). The absorbed light kicks an electron off the appropriate level. depending on the material it then cascades down several, $\endgroup$ – anna v Oct 16 '13 at 5:51
  • $\begingroup$ levels, turning the energy of one photon into many soft ones and then heat ( photons absorbed to give higher vibrational energy in solids for example). The number of electrons and protons changes the potential and therefore the energy levels which determine the energy of the photons. Anyhow, I cannot answer in depth in comments all these questions. You need to read up on atomic physics. $\endgroup$ – anna v Oct 16 '13 at 5:56
  • $\begingroup$ Do you have any resources you recommend? Preferably in a somewhat friendly format. Thank you for clearing a lot of this up though. $\endgroup$ – keroro Oct 16 '13 at 6:16

Color is a purely psychophysical creation of the human eye, in response to EM radiation in the single octave from 400 to 800nm wavelength (air), but most older people have negligible response from 700 to 800 nm. "Light" is likewise a creation of the human eye. That's why we use separate units of measurement for it; lumens, candelas etc.

Atoms etc have NO color. They create EM radiation energy,at various wavelengths, only some of which evoke the human eye "light" response. Scientists need to watch their language, and use correct terminology, if they want to convey useful information, and not confuse students, with loose language.

"Light" by definition, IS visible. It IS NOT UV or IR.


Correct me if I am wrong: What frequencies are reflected/absorbed depends upon the structure of the atom (which, as quantum mechanics has taught us, is something really hard to understand). So to know what particular property of the atom decides which frequency/wavelength it should absorb/reflect is very hard. As for electrons and other subatomic particles - they do not possess any color, so to speak, since visible light has a wavelength too huge to reflect off these particles. That is why you must use X-rays and electron microscopes to observe atoms (and we still can't see subatomic particles, though I really do wish we could - finally the whole uncertainty thing will have a whole new angle to it, don't you think?).


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