Complex sentence: To make an actual laser, a beam has to pass through mass quantities of laser medium by bouncing back and forth between mirrors placed at opposing ends, then the light beam exits through one of the mirrors which is more transparent than the other.
Split into simpler sentences: To make an actual laser, a beam has to pass through mass quantities of laser medium by bouncing back and forth between mirrors placed at opposing ends. Then the light beam exits through one of the mirrors which is more transparent than the other.