Answer to Question #35582 in Atomic and Nuclear Physics for dinesh
why laser beam is parallel in ruby laser?
The finely polished ends of the crystal of ruby were silvered; one end completely, the other only partially. Photons traveling parallel to the axis of the tube (crystal) will start a cascade of photon emission while photons traveling in other directions pass out of ruby. Photon beam parallel to the axis of tube grows in strength and some of it bursts through partial reflector and serves as output laser beam.