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The Laser

Who invented it ?

"The ideas that led to the invention of the laser were discovered by Albert Einstein in 1916. But it was not until 1953 that these ideas were put into use. The first laser was actually called a maser, because it used microwaves. (This is the same type of energy that is used in a microwave oven). Charles Townes and two of his students made the first maser, but it would only work for short amounts of time. Two scientists from the Soviet Union (Russia) named Nikolay Basov and Aleksandr Prokhorov figured out how to make the maser stay on. These three men won a Nobel Prize in 1964 for their discovery.

The first idea for a laser came from Charles Townes and Arthur Schawlow. Gordon Gould worked on the idea and wrote them down in a paper in 1959. This paper was the first time the word laser was used.

The first working laser was invented by Theodore Maiman and demonstrated on May 16, 1960."

http://en.wikibooks.org/wiki/Wikijunior:How_Things_Work/LaserImage Added

Definition :

 A Laser is device that creates and amplifies electromagnetic radiation of a specific frequency through the process of stimulated emission. The radiation emitted by a laser consists of a coherent beam of photons, all in phase and having the same polarization.

A laser emits a thin, intense beam of nearly monochromatic visible or infrared light that can travel long distances without diffusing. Most light beams consist of many waves traveling in roughly the same direction, but the phases and polarizations of each individual wave (or photon) are randomly distributed. In laser light, the waves are all precisely in step, or in phase, with each other, and have the same polarization. Such light is called coherent. All of the photons that make up a laser beam are in the same quantum state. "Lasers produce coherent light through a process called stimulated emission. The laser contains a chamber in which atoms of a medium such as a synthetic ruby rod or a gas are excited, bringing their electrons into higher orbits with higher energy states.
When one of these electrons jumps down to a lower energy state (which can happen spontaneously), it gives off its extra energy as a photon with a specific frequency. But this photon, upon encountering another atom with an excited electron, will stimulate that electron to jump down as well, emitting another photon with the same frequency as the first and in phase with it.

This effect cascades through the chamber, constantly stimulating other atoms to emit yet more coherent photons. Mirrors at both ends of the chamber cause the light to bounce back and forth in the chamber, sweeping across the entire medium. If a sufficient number of atoms in the medium are maintained by some external energy source in the higher energy statea condition called population inversionthen emission is continuously stimulated, and a stream of coherent photons develops. One of the mirrors is partially transparent, allowing the laser beam to exit from that end of the chamber. 

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Schematic:

How does it work ?

A laser is an optical oscillator, which is can be made out of a solid, liquid or gas. It contains two with mirrors at both ends: one is totally reflecting and the other only partially. To make it work, the material is excited or "pumped," with an energy source such as light or electricity.

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