What is meant by directionality? What is it due to?
As we know that in case of stimulated emission, atoms in an upper energy level are triggered or stimulated in phase by an incoming photon of a specific energy. The incident photon must have an energy corresponding to the energy difference between the upper and lower states. The emitted photons have the same energy as incident photon. These photons are in phase with the triggering photon and also travel in its direction. This leads to directionality in case of a laser. Further, the mirrors placed at opposite ends of a laser cavity enables the beam to travel back and forth in order to gain intensity by the stimulated emission of more photons at the same wavelength, which results in increased amplification due to the longer path length through the medium. The multiple reflections also produce a well-collimated beam, because only photons traveling parallel to the cavity walls will be reflected from both mirrors. If the light is the slightest bit off axis, it will be lost from the beam. The
