why can an electron not absorb the whole energy of a photon?
Answers & Comments
AJKR The premise of this question is actually false. First, I'll need to rewrite it slightly. "Partial absorption" of a photon doesn't make sense. Photons are particles - you can have 0 photons or 1 photon but not half a photon. But you can change this is "Partial absorption of the photon's energy" and then it is kosher. So, the modified question reads
"Why does a photon transfer all its energy to an electron (and disappears) rather than transferring part of its energy and continuing to exist?"
The premise is untrue - the photon CAN transfer part of its energy to the electron and continue to exist. An example of this is Compton scattering, where a gamma ray (a type of photon) transfers enough energy to the electron to ionize and move it out of the atom's potential well.
On the other end of the spectrum, consider the passage of light through a transparent material. Here the light scatters off the electrons in the material elastically (in a way that transfers no energy to the electrons). So the photons interact with the electrons and transfer NO energy. There are also types of elastic scattering such as Rayleigh scattering which I don't know enough about to describe.
Often the process you describe happens, where the photon is completely absorbed by the electron. This is an electron that is trapped by the atom and can only exist in certain energy levels. This is not because energy is quantized, as César Tomé-López says (perhaps I misunderstood him). Any energy is possible in principle - just not in the atom. The quantization is because of the nature of the Coulomb force coupling the electron to the nucleus.
So, you might ask, why can't the photon transfer part of its energy to the electron (enough to transition between two energy levels) and then travel on with reduced energy? After all, it can transfer enough energy to completely ionize the electron, why not just enough to excite the electron between two energy states? I don't think this is a simple question, or if it is I do not know its simple answer. I think it might be due to conservation of angular momentum.
Another possibility is the intricacy of a photon scattering event. Although I have talked as though the photon and electron simply exchange energy, what actually happens is that the photon and electron join into a higher energy electron that then reemits a photon. Perhaps this intermediate electron is forced to exist in one of the quantized energy levels. I am unsure about this, because generally these intermediate virtual particles can do crazy things like violate energy conservation for a brief period of time.
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Gurbir
sorry. but a photon imparts some of its energy to the electron during compton scattering and a new photon of lesser energy is released(lesser frequency too). therefore the question isn't wrong
Gurbir
because electron absorbs full energy of photon in photoelectric effect. but it can absorb only partial energy in compton effect when it is isolated.
Answers & Comments
The premise of this question is actually false. First, I'll need to rewrite it slightly. "Partial absorption" of a photon doesn't make sense. Photons are particles - you can have 0 photons or 1 photon but not half a photon. But you can change this is "Partial absorption of the photon's energy" and then it is kosher. So, the modified question reads
"Why does a photon transfer all its energy to an electron (and disappears) rather than transferring part of its energy and continuing to exist?"
The premise is untrue - the photon CAN transfer part of its energy to the electron and continue to exist. An example of this is Compton scattering, where a gamma ray (a type of photon) transfers enough energy to the electron to ionize and move it out of the atom's potential well.
On the other end of the spectrum, consider the passage of light through a transparent material. Here the light scatters off the electrons in the material elastically (in a way that transfers no energy to the electrons). So the photons interact with the electrons and transfer NO energy. There are also types of elastic scattering such as Rayleigh scattering which I don't know enough about to describe.
Often the process you describe happens, where the photon is completely absorbed by the electron. This is an electron that is trapped by the atom and can only exist in certain energy levels. This is not because energy is quantized, as César Tomé-López says (perhaps I misunderstood him). Any energy is possible in principle - just not in the atom. The quantization is because of the nature of the Coulomb force coupling the electron to the nucleus.
So, you might ask, why can't the photon transfer part of its energy to the electron (enough to transition between two energy levels) and then travel on with reduced energy? After all, it can transfer enough energy to completely ionize the electron, why not just enough to excite the electron between two energy states? I don't think this is a simple question, or if it is I do not know its simple answer. I think it might be due to conservation of angular momentum.
Another possibility is the intricacy of a photon scattering event. Although I have talked as though the photon and electron simply exchange energy, what actually happens is that the photon and electron join into a higher energy electron that then reemits a photon. Perhaps this intermediate electron is forced to exist in one of the quantized energy levels. I am unsure about this, because generally these intermediate virtual particles can do crazy things like violate energy conservation for a brief period of time.