As the outer electrons are turned up to higher energy levels the chromium atoms begin excitation. These electrons fall back into an intermediate level and then fall slowly into a lower level. At this stage the photons begin to emit red light. Instead of normal conditions, in this case, more atoms are in the upper energy levels rather than in lower ones. Figure 2 A Ruby Crystal Laser.
Coherent light is produced since there are more excited atoms than unexcited ones.
Instead of the change from one photon to absorbtion, there is a change from one photon to two photons which is called stimulated emission. These photons that were stimulated by incident photons will be identical in frequency, phase, and direction to the incident photons. These photons stimulate other atoms which produces coherent light. Every time the photoflash is fired the light travels along the crystal axis and is reflected off of many coated mirrors. While one mirror is totally reflecting this light another is partially reflecting causing light to build up and create an intensity.