Published online by Cambridge University Press: 24 October 2008
When an atomic system emits or absorbs radiation, the frequency of the radiation is connected with the change of energy of the system by Bohr's frequency condition δE = hv. This equation is usually applied to the transition of an atom from one stationary state to another, the frequency of the radiation being measured in a frame of reference in which the atom as a whole is at rest, both before and after the transition. It has been pointed out by Bohr that difficulties arise when no natural frame of reference presents itself in which to measure the frequency, which is the case for non-isolated systems, or even for isolated systems when the process of radiation is connected with a change of momentum.
* Camb. Phil. Soc. Proc. (Supplement), “On the Applications of the Quantum Theory to Atomic Structure,” pp. 28, 32.Google Scholar
† Phys. Zeits. 23, p. 301 (1922).Google Scholar