However, it is readily observed that if the radiation from a gas discharge tube is passed through another tube of the same gas, then absorption of radiation occurs. This may be attributed to the finite widths of the spectral lines. Most of this width is due to doppler broadening associated with the thermal motion of the atoms. However, if the photon energy is large enough, as for example in a gamma ray emitted from an atomic nucleus, then the recoil becomes more pronounced and the effect is observable. It was first seen in 1951 using the Mossbauer effect.
Similar considerations play a role in the laser cooling of atoms.
Since an atom moving away from the laser light will absorb at a slightly different frequency it will not absorb the same light and so will not be speeded up in the opposite direction. Though the atom will recoil when it reemits the light, that reemission will be in all directions. Hence, on average, atoms moving towards the laser will be slowed down, in that direction. This mechanism has been used to cool atoms to extremely low temperatures.