Let's summarize the ways in which light can interact with matter... individual atoms, molecules, or even metallic solids.
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(a) A photon can be absorbed by an
atom or molecule, which is excited to a higher energy level. (b)
An atom or molecule in an exited state can de-excite to a lower
energy level by emitting a photon. (c) A photon passing through
an atom or molecule in an excited state can trigger the
de-excitation of the level by photon emission, if the incoming
photon has the same energy as the gap between the two energy
levels. (d) A photon can scatter from an atom or molecule, going
off in a different direction, if it has the right frequency,
with the atom or molecule being unaffected. (e) A photon passing
through an atom or molecule can excite or de-excite a bound
electron, changing the photon's frequency.
(f) A photon incident on a metal surface can knock out an electron from the surface. (g) A photon incident on an atom or molecule where an electron is in a bound state can be absorbed by knocking out the electron. (h) A photon can scatter elastically from a single electron, knocking it out of the atom or molecule as it passes through. (i) A photon in the Coulomb field of a heavy nucleus can create a particle-antiparticle pair, if it has a kinetic energy equal to or larger than the combined mass of the pair.
The existence of the phenomenon of stimulated emission was first proposed by Albert Einstein in the middle of the 1910s. It took a while to confirm its existence, but in the last half of the 20th Century the concept was widely applied in creating amplifiers of electromagnetic radiation--- first masers, in 1953, which amplified microwaves, and then lasers, in 1960, which amplified visible light.
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It is interesting that ammonia masers exist naturally in clouds of gas in interstellar space, and astrophysicists have found many examples!
Lasers, which operate in the visible light region, have a handicap that does not apply to masers. Namely, they have to be pumped. That means that the atoms or molecules or other quantum systems used in the stimulated emission have to be artificially placed into an excited state, so that stimulated emission can take place. There are many different ways to do this, fortunately.
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Technological applications of lasers are almost limitless, but one of the most scientifically important uses is the creation of huge laser interferometer systems to detect gravitational radiation. Observation of gravitational radiation emitted by violent events in the universe such as collisions of two black holes, a neutron star and a black hole, or two neutron stars, has given us an entirely new way to view very common astrophysical processes occurring all over our universe!
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