The laser is a device that a beam of light that is both scientifically and practically of great use because it is coherent light. The beam is produced by a process known as stimulated emission, and the word "laser" is an acronym for the phrase "light amplification by stimulated emission of radiation."
Lasers consist of several components. A few of the many things that the so-called active medium might consist of atoms of a gas, molecules in a liquid, and ions in a crystal. Another component consists of some method of introducing energy into the active medium, such as a flash lamp for example. Another component is the pair of mirrors on either side of the active medium which consists of one that transmits some of the radiation that hits it. If the active component in the laser is a gas laser than each atom is characterized by a set of energy states, or energy levels, of which it may consist. An example of the energy states could be pictured as a unevenly spaced ladder which the higher rungs mean higher states of energy and the lower rungs mean lower states of energy. If left disturbed for a long time the atom will reach its ground state or lowest state of energy. According to quantum mechanics there is only one light frequency that the atom will work with. There are three ways that the atom can deal with the presence of light either it can absorb the light, or spontaneous emission occurs, or stimulated emission occurs. This means that if the atom is at its lowest state that it may absorb the light and jump to its high state and emit extra light while doing so. The second thing it may do is if it is at its highest state it can fall spontaneously to its lower state thus emitting light. The third way is that the atom will jump from its upper state to its lower state thus emitting extra light. Spontaneous emission is not effected by light yet it is rather on a time scale characteristic of the states involved. That is called the spontaneous lifetime. In stimulated emission the frequency of the light is the same as the frequency of the light that stimulated it.