All matter of is made up of atoms. Atoms consist of a tiny nucleus and – in comparison to the nucleus – a huge electron sheath. The electrons in the sheath race around the nucleus in different orbits representing different levels of energy. If an electron in a certain energy level is hit by a photon with a corresponding energy content this will stimulate the electron to emit another photon. The second, emitted photon will have the same energy and direction as the original photon. This is known as stimulated emission, the principle behind the laser.

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