Why does hydrogen gas absorb energy to excite electrons if it then emits photons?

[gracy]

If the hydrogen gas is supposed to emit photons when the electrons undergo excitation, then why did the gas absorb energy earlier and got the electrons excited?

 

[DrClaude]

Absorption of photons, collisional excitation, etc.

 

[mathman]

Electrons absorb photons when excited. Photons are emitted when electrons lose energy.

 

[bhobba]

The real answer requires QFT and takes into account the quantum vacuum:
http://www.physics.usu.edu/torre/3700_Spring_2014/What_is_a_photon.pdf

Thanks
Bill