1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of the atoms in the ruby crystal to higher energy levels.
 2. At a specific energy level, some atoms emit particles of light called photons. At first the photons are emitted in all directions. Photons from one atom stiumulate emission of photons from other atoms and the light intensity is rapidly amplified.
 3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification.
 4. The photons leave through the partially silvered mirror at one end. This is laser light.

This table was extracted from a laser basics web page developed at Lawerence Livermore National Laboratory.