# Does an atom gain mass when it absorbs a photon?

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...

• Your assumptions are correct. – my2cts Aug 9 '18 at 20:57

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.
• $m = h \nu$ / $c^2$ – Neoheurist Aug 9 '18 at 21:58