2
$\begingroup$

I understand the at rest a photon has no mass, but it has energy. So when a photon is absorbed by an atom the atom gains the energy of the photon. This captured energy raises the mass of the atom by some quantity... I'm guessing that the frequency of the photon determines the amount of mass added to the atom...

An example for a given frequency would be helpful for me to understand.

Any help would be appreciated, my cat really wants to know and I'm running out of games to distract him... he's very demanding...

$\endgroup$
  • 1
    $\begingroup$ Your assumptions are correct. $\endgroup$ – my2cts Aug 9 '18 at 20:57
2
$\begingroup$

Sure, the atom will gain mass. But that extra energy puts the atom into an unstable state, so the atom will radiate that energy away again in a short time, and so the mass gain is only temporary.

We can calculate the mass gain using $E = mc^2$ and $E = h \nu$, where $\nu$ is the frequency of the photon, and $h$ is Planck's constant.

$\endgroup$
  • 1
    $\begingroup$ $m = h \nu$ / $c^2$ $\endgroup$ – Neoheurist Aug 9 '18 at 21:58
  • 1
    $\begingroup$ @Neuheurist Correct. But I didn't want to write it like that in case someone interpreted it as the mass of the photon, which is zero, as you said. $\endgroup$ – PM 2Ring Aug 9 '18 at 22:01
  • $\begingroup$ It's the mass of the photon's energy... $\endgroup$ – Neoheurist Aug 9 '18 at 22:03
  • $\begingroup$ @Neoheurist Sort of. You can call it the effective mass. Old books would call it the relativistic mass, but we try to avoid using that concept these days, since it can be misleading and confusing. $\endgroup$ – PM 2Ring Aug 9 '18 at 22:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.