D O C . 6 8 E X P E R I M E N T O N L I G H T E M I S S I O N 2 5 5
68. “On an Experiment Concerning
the Elementary Process of Light Emission”
[Einstein 1922a]
Submitted 8 December 1921
Published 5 January 1922
In: Königlich Preußische Akademie der Wissenschaften (Berlin). Sitzungsberichte (1921):
There is no doubt that the radiation emitted in an elementary process (in the quan-
tum-theoretic sense) from an atom at rest is monochromatic. If the emitting particle
has a velocity relative to the coordinate system, the radiation emitted in different
directions during the elementary process is supposed to have different frequencies.
If v is the velocity of the particle, the frequency of emission during the elemen-
tary process as seen from the particle, then one should have in first-order approxi-
, (1)
where is the angle between the direction of the moving particle and the direction
of emission considered.
On the other hand, if one considers Bohr’s emission condition
that connects the energy change of the atom with the emitted frequency, which is
so fundamental in quantum theory, then one is inclined to assign a unique frequen-
cy to every elementary emission process, and also to the process of emission from
a moving atom.
The question whether the consequence, following from the undulation theory or
the one suggested by quantum theory (even though not demanded by it), is the cor-
rect one can be decided by the following experiment (see the adjacent sketch).
ν ν
-- -
cos +

E2 E1 hν1 =
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