A laser is a device that produces a beam of coherent light. Every laser has a source of atoms called an active medium, which can be a gas, liquid, solid or semi conductor.

The atoms in the medium are excited to meta–stable states by an external source of energy. When most of the atoms in the medium are excited, a single photon from an atom that undergoes de–excitation can start a chain reaction. This photon strikes another atom, stimulating it into emission, and so on and producing coherent light.

There are many different types of lasers. Lasers are commonly designated by the type of lasing material employed:

• Solid–state lasers have lasing material distributed in a solid matrix (such as the ruby or neodymium:yttrium–aluminum garnet "Yag" lasers).
• Gas lasers (helium and helium–neon, HeNe) have a primary output of visible red light. CO₂ lasers emit energy in the far–infrared, and are used for cutting hard materials.
• Dye lasers use complex organic dyes, in liquid solution or suspension as lasing media. These are tunable over a broad range of wavelengths.
• Semiconductor lasers, sometimes called diode lasers. These electronic devices are generally very small and use low power. These are used in some laser printers or CD players.

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