A laser is a device that controls the way that energized atoms release photons.
"Laser" is an acronym for light amplification by stimulated emission of radiation,
which describes very succinctly how a laser works.
Although there are many types of lasers, all have certain essential features. In a
laser, the lasing medium is .pumped. to get the atoms into an excited state.
Typically, very intense flashes of light or electrical discharges pump the lasing
medium and create a large collection of excited-state atoms (atoms with
higher-energy electrons). It is necessary to have a large collection of atoms in
the excited state for the laser to work efficiently. In general, the atoms are
excited to a level that is two or three levels above the ground state. This
increases the degree of population inversion. The population inversion is the
number of atoms in the excited state versus the number in ground state.
Once the lasing medium is pumped, it contains a collection of atoms with some
electrons sitting in excited levels. The excited electrons have energies greater
than the more relaxed electrons. Just as the electron absorbed some amount of
energy to reach this excited level, it can also release this energy. the electron
can simply relax, and in turn rid itself of some energy. This emitted energy comes
in the form of photons (light energy). The photon emitted has a very specific
wavelength (color) that depends on the state of the electron's energy when the
photon is released. Two identical atoms with electrons in identical states will
release photons with identical wavelengths.
Laser light is very different from normal light. Laser light has the following
properties:
* The light released is monochromatic. It contains one specific wavelength of
light (one specific color). The wavelength of light is determined by the amount of
energy released when the electron drops to a lower orbit.
* The light released is coherent. It is .organized. -- each photon moves in
step with the others. This means that all of the photons have wave fronts that
launch in unison.
* The light is very directional. A laser light has a very tight beam and is
very strong and concentrated. A flashlight, on the other hand, releases light in
many directions, and the light is very weak and diffuse.
To make these three properties occur takes something called stimulated emission.
This does not occur in your ordinary flashlight -- in a flashlight, all of the
atoms release their photons randomly. In stimulated emission, photon emission is
organized.
The photon that any atom releases has a certain wavelength that is dependent on the
energy difference between the excited state and the ground state. If this photon
(possessing a certain energy and phase) should encounter another atom that has an
electron in the same excited state, stimulated emission can occur. The first photon
can stimulate or induce atomic emission such that the subsequent emitted photon
(from the second atom) vibrates with the same frequency and direction as the
incoming photon.
The other key to a laser is a pair of mirrors, one at each end of the lasing
medium. Photons, with a very specific wavelength and phase, reflect off the mirrors
to travel back and forth through the lasing medium. In the process, they stimulate
other electrons to make the downward energy jump and can cause the emission of more
photons of the same wavelength and phase. A cascade effect occurs, and soon we have
propagated many, many photons of the same wavelength and phase. The mirror at one
end of the laser is "half-silvered," meaning it reflects some light and lets some
light through. The light that makes it through is the laser light.
From: How Stuff Works