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My book says that when a photon carrying a certain amount of energy hits an electron, that gets excited and goes on an higher energetic level, absorbing the energy of the photon. When it comes back to its main energetic level it emits another photon.

  1. Is this second photon having the same frequency as the first one and therefore same energy? If so, why there are black lines on the absorption spectogram?

  2. Is this second photon having a lower frequency and therefore less energy (and then a different colour on the spectogram)?

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Generally speaking, the emitted frequency is not necessarily the same as that absorbed but let's suppose that it is.

As the white light (composed of all the different frequencies) encounters the material (the red box, below), certain frequencies are absorbed and then re-emitted but in random directions. So they're scattered. Therefore, far fewer photons of those particular frequencies are observed which gives rise to the dark spectral lines.

enter image description here

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  • $\begingroup$ So if we could manage to detect all the photons emitted by the red box, even the scattered ones, we would have a full complete spectrum without dark lines? Also, you said that the emitted frequency is not necessarily the same as that absorbed. Well, what exactly determines the frequency of the emitted photon? $\endgroup$ – user3501165 Sep 6 '14 at 14:29
  • $\begingroup$ @user3501165 Yes, in principle. $\endgroup$ – lemon Sep 6 '14 at 14:36
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    $\begingroup$ In addition to the scattering factor: a high energy photon can be absorbed, jumping the electron up multiple levels. The electron can then bounce down randomly through the intervening levels, emitting various low energy photons $\endgroup$ – DJohnM Sep 6 '14 at 19:21

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