This applet models the interaction between electrons (shown as blue dots) and photons (red squiggles). Four processes are modelled: absorption, stimulated emission, spontaneous emission and pumping; the frequency of occurrence of these processes may be set using the text fields provided.
The absorption rate is the probability that a photon will be absorbed by any one electron; similarly for the stimulated emission rate. The other two rates, for spontaneous emission and pumping are the probability per time interval T of those events occurring, where T is the time taken for a photon to travel from one side of the display to the other.
The population inversion (proportion of electrons at the upper level) is displayed in the top corner of the applet. Using the "inversion" field, this value may be set. When the "apply inversion" button is clicked, the electrons will be randomly distributed according to the specified inversion.
In addition, an inversion of 0 or 1 may be set using the "none excited" or "all excited" buttons.