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Clovis
- 17
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I wonder of someone could help me understand something about emission and absorption of photons by atoms. I thought I understood the basics of this topic but yesterday a thought experiment occurred to me that seemed to violate conservation of energy.
Imagine two stationary hydrogen atoms. Atom A, on the left, is in an excited state. Atom B, on the right, is in the ground state. Atom A returns to the ground state by emitting a photon. I recall reading that emission spectra had the same frequencies as absorption spectra, so if atom B absorbs the photon from atom A, then I assume atom B should enter into the same excited state that atom A had been in. From a conservation of energy standpoint, atom B has gained all the energy that atom A has lost, so there is no net energy change to the system. All the energy of atom A’s excited state has been “spent” in raising atom B to the same excited state.
But I also recall reading that photons have momentum. That implies to me that atom A recoiled to the left when it emitted the photon and that atom B recoiled to the right when it absorbed the photon. Since both previously stationary atoms now are in motion, they have gained some kinetic energy. My question is where did that extra energy come from? It seems to me that the kinetic energy could not have come from atom A’s excited energy, because all of that energy was used to raise atom B to the same excited state that atom A was initially in.
I know conservation of energy can’t be violated, but where has my reasoning gone astray? Any help would be greatly appreciated. Clovis
Imagine two stationary hydrogen atoms. Atom A, on the left, is in an excited state. Atom B, on the right, is in the ground state. Atom A returns to the ground state by emitting a photon. I recall reading that emission spectra had the same frequencies as absorption spectra, so if atom B absorbs the photon from atom A, then I assume atom B should enter into the same excited state that atom A had been in. From a conservation of energy standpoint, atom B has gained all the energy that atom A has lost, so there is no net energy change to the system. All the energy of atom A’s excited state has been “spent” in raising atom B to the same excited state.
But I also recall reading that photons have momentum. That implies to me that atom A recoiled to the left when it emitted the photon and that atom B recoiled to the right when it absorbed the photon. Since both previously stationary atoms now are in motion, they have gained some kinetic energy. My question is where did that extra energy come from? It seems to me that the kinetic energy could not have come from atom A’s excited energy, because all of that energy was used to raise atom B to the same excited state that atom A was initially in.
I know conservation of energy can’t be violated, but where has my reasoning gone astray? Any help would be greatly appreciated. Clovis
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