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The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. The easiest laser model to understand is the two level system. In a two level system, the particles have only two availible energy levels, separated by some energy difference which is typically referred to in terms of the photon energy, hv0. These two levels are generally referred to as the upper and lower laser states. When a particle in the upper state interacts with a photon matching the energy separation of the levels, the particle may decay, emitting another photon with the same phase and frequency as the incident photon. Thus we have gotten two photons for the price of one. This process is known as stimulated emission.
A laser is a device that intensifies light. Put in simple terms, the atoms of the laser material serve as energy storehouses. Atoms can only store energy in very specific “packet” sizes. These energy portions are transported in the form of photons. Light of a specific wavelength is created by the photons, which transport a certain, predetermined energy quantity.

If an atom has free storage space for a packet of energy of a certain size and a photon flies by with an energy packet of that exact size, the atom can absorb and store the energy packet the photon is transporting. This is referred to as absorption.

Once an atom has stored an energy packet, at some point it will emit this energy in the form of a photon. This photon will fly in an undetermined direction. The process described here is called spontaneous emission.