When an electron absorbs a photon it leaps to a higher energy level, what exactly happens when an electron absorbs a photon? By the mass-energy equivalence doesn't that changes the electron's mass and thus alters it?

  • $\begingroup$ Like all seemingly simple questions, this one can be answered on many different levels. On what level are you asking? What exactly do you mean by absorbing? Have you read about Compton Scattering? $\endgroup$ – seb Dec 8 '13 at 6:49
  • $\begingroup$ I am not really an expert in this field, but I would like to say my opinion. Up to my knowledge, absorption of photon does not mean photon being absorbed by the electron. Photon will be under high speed (determined by the frequency of incident radiation) which causes tangential displacement of electron to next higher energy level, just like if you hit a marble to another marble like particle, there will be displacement of electron to next higher energy level. $\endgroup$ – Immortal Player Dec 8 '13 at 14:26

These days we treat the mass of a particle is invariant (rest mass) to avoid confusion, but as the electron gains kinetic energy, it's energy goes from being $mc^2$ to $\gamma\, m c^2$ where

$$\gamma =\frac{1}{ \sqrt{1-\frac{v^2}{c^2}}}$$

is a relativistic factor arising from the electron's speed $v$. If you tried to further accelerate the electron, you would find that it behaves as though it's mass were $\gamma m \gt m $, but this is an artifact of applying Newtonian expectations to relativistic effects.

Note that you can expand $E = \gamma m c^2$ as a series, and if $v \ll c$ then you only need to keep the first few terms: $$E = m c^2 + \frac{1}{2} m v^2 + \dots$$


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