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What is the property of the molecules of the material that causes it to behave differently when it interacts with light. Is it the frequency of the material?

I was thinking that the resonant frequency and the frequency of oscillation of the material has a role to play in it

Swaroop Joshi
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  • Yes, it's kind of similar to what has been answered here: Which is what I was looking for

    But the actual answer maybe explained by QED(as mentioned in the discussion), etc and I don't want to dive so deep, yet. Maybe some time later

     – Swaroop Joshi Dec 15 '19 at 13:49

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I believe you are right, that the single most important characteristic is the energy level spectrum of the solid. However, in my simple minded picture there is a second important characteristics: The coupling to other electronic states. The argument is as follows:

  • If the solid does not provide an energy level which is "close" to resonance with the incoming frequency of the light, the interaction strength is "weak". Therefore, the solid refracts the light. Still, the index of refraction $n$ is a function of the energy spectrum of the solid.

  • If the solid provides an electronic transition which is "close" to resonance with the incoming frequency of the light, the light will be absorption. Now, whether or not the light will be re-emitted depends on the coupling of the resonant energy level with the other energy levels of the solid:

    • If the excited electron couples "strongly" (=transition strength) only to the initial state of the electron, the excited electron will most probably re-emit the photon. Thus, we will get reflection -- I omit any coherence argument, so that the text remains simple.
    • In contrast, if the excited electron state couples "strongly" also to other electronics states (not only the initial state) the probability to re-emit the photon becomes "small". Thus the photon is absorpted. The two simple cases to obtain absorption are, if there exists

    (a) "many" states which posses approx. the same transition strength as the initial state or

    (b) one state which has a "much" larger transition strength than the initial state.

Semoi
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  • So the brightness of the object can roughly be dependant on the probability of coupling to the initial state? Of course, there might be other factors – Swaroop Joshi Dec 20 '19 at 19:38
  • What do you mean by "coupling to the initial state"? Please be more verbose. – Semoi Dec 20 '19 at 19:43
  • I mean to ask that, if it 'couples strongly to the initial state'(the electron would more often fall back to the initial state than any other state), would the brightness be more? Since it would more often emit the photon. – Swaroop Joshi Dec 20 '19 at 19:53
  • Yes, if the excited electron state couples "very strongly" to the initial electron state and only "weakly" to other electron state, the electron would absorb and reemit the photon. – Semoi Dec 20 '19 at 20:30