My understanding is: when light hits an object, it can do one of three things: transmit through the object, be absorbed into the object, or reflect from the object.

However, I have also learned that when light hits atoms, it can excite electrons, leading to another emission of light as the electrons go back down to their ground states.

The first paragraph, while intuitive, does not seem to be logically explained by the second. Can someone please help me understand the first in terms of the concepts of the second?

Specifically, is absorption of light exciting an electron with more energy than necessary, and some energy remains in the atom, or is the entire energy of the photon absorbed?

Is reflection just hitting the atom with a photon that is just the right amount of energy to get it excited, so no net energy is gained?

Thank you.

  • $\begingroup$ Another way light and matter can interact is scattering which is similar to reflection in some ways but not the same. $\endgroup$ – M. Enns Apr 16 '16 at 3:13
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    $\begingroup$ This was cross-posted in the chemistry section first chemistry.stackexchange.com/q/49551 $\endgroup$ – MaxW Apr 16 '16 at 4:27
  • $\begingroup$ Please see our guide on writing good titles. $\endgroup$ – user10851 Apr 16 '16 at 18:32

Would you accept "It's complicated..."? The first problem is that even though (atomic) solids are composed of atoms, they don't behave like the atoms they are made from when hit by light. For that you have to have a dilute mono-atomic gas. I can't think of a single element at the moment (someone please help me out!) that has a gaseous phase comprised of single atoms at normal conditions, and I can't recall which (any) will interact with visible light by electronic transitions from their ground state. Most solids do not interact by simple electronic transitions, either, but what you see are molecular excitations or bulk electronic effects (in metals) instead. There are rare exceptions in diluted systems where individual fluorescent atoms are embedded in a solid matrix, but that just makes things more complicated, still, as these atomic probes are seeing the local fields in the solid.

If you want to directly observe true atomic transitions, you have to get an element to an atomic gas state, first, and then you have to excite it either with ultraviolet light, by collision or with an electric discharge to get into an excited state from which there are visible and near IR transitions.

In comparison, solids and liquids made of molecules can show interesting molecular excitations which give them their colors that can be seen in absorption and reflection. The shiny appearance of metals is based on the free delocalized electrons, so that's a totally different mechanism, again.

This only scratches the range of possibilities. There are single trapped electrons (called "color centers", e.g. visible in blue fluorescing diamonds), inner shell electrons in fluorescent rare earth metals and a plethora of more complex mechanism that enable light-matter interaction in the solid phase.

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    $\begingroup$ I think Helium in gaseous phase is monatomic. Helium doesn't combine with anything in standard situations, not even itself. $\endgroup$ – K7PEH Apr 16 '16 at 5:10
  • $\begingroup$ @K7PEH: Hah... of course I had to miss the obvious - the noble gases. Thanks! :-) Any idea for electronic transitions from the ground state in the visible? Chlorine/bromine (fluorine?) are colored, but they are both molecular, as far as I know. I don't think any of the other noble gases have visible transitions, either. Mercury vapor has its first transition in the UV and everything else needs higher temperature to have significant vapor pressure, I believe. $\endgroup$ – CuriousOne Apr 16 '16 at 5:32
  • $\begingroup$ Your question on electronic transitions from visible light photons is a good question for my daughter's closest friend who holds a PhD in Chemistry and her graduate/thesis and post-doc work was all using lasers to effect various chemical transitions or reactions of sort. Unfortunately, I know very little about her research. She currently works for the government in D.C. But, as for me, I might have exhausted my knowledge on the subject other than knowledge that UV light was the lowest frequency to ionize atoms in most (maybe not all) cases. $\endgroup$ – K7PEH Apr 16 '16 at 14:43

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