An atom in an excited state gives off energy by emitting a photon, a quantum of electromagnetic radiation, according to Bohr’s second postulate. Although Bohr’s specific model of the atom has been vastly extended and incorporated into models based on a different approach , this postulate is still valid.

Atoms can acquire internal energy, that is, be brought to an excited state, in many ways. In the Franck–Hertz experiment, inelastic collisions provided the energy; in a cool gas displaying a dark-line spectrum, it is the absorption of photons; in a spark or discharge tube, it is collisions between electrons and atoms. There are other mechanisms as well.

Once an atom has acquired internal energy, it can also get rid of it in several ways. An atom can give up energy in inelastic collisions, or (as discussed above) it can emit energy as electromagnetic radiation. There are many different kinds of inelastic collisions; which one an atom undergoes depends as much on its surroundings as on the atom itself.

There are also two different ways an atom can emit radiation. Spontaneous radiation is one of them. At some random (unpredictable) moment, the previously excited atom emits a photon (of frequency ν) and changes its state to one of lower energy (by an amount ΔE).

If, however, there are other photons of the appropriate frequency ( f = ΔE/h) in the vicinity, the atom may be stimulated to emit its energy. The radiation emitted is at exactly the same frequency, polarization, and phase as the stimulating radiation. That is, it is exactly in step with the existing radiation. In the wave model of light, you can think of the emission simply increasing the amplitude of the oscillations of the existing electromagnetic field within which the emitting atom finds itself.

A collection of atoms stimulating one another to emit radiation behaves much like an antenna. You can think of the electrons in the different atoms as simply vibrating in step just as they do in an ordinary radio antenna, although much, much faster.

Usually atoms emit their energy spontaneously long before another photon comes along to stimulate them. Most light sources therefore emit incoherent light, that is, light made up of many different contributions, differing slightly in frequency, out of step with each other, and randomly polarized.

Usually, most of the atoms in a group are in the ground state. Light that illuminates the group is more likely to be absorbed than to stimulate any emission, since it is more likely to encounter an atom in the ground state than in the appropriate excited state. But suppose conditions are arranged so that more atoms are in one of the excited states than are in the ground state. (Such a group of atoms is said to be inverted.) In that case, light of the appropriate frequency is more likely to stimulate emission than to be absorbed. Then an interesting phenomenon takes over.

Stimulated emission becomes more probable the more light there is around. The stimulated emission from some atoms therefore leads to a chain reaction, as more and more atoms give up some of their internal energy to the energy of the radiation. The incident light pulse has been amplified. Such an arrangement is called a laser (light amplification by stimulated emission of radiation).

Physicists and engineers have developed many tricks for producing “inverted” groups of atoms, on which laser operation depends. Exactly what the tricks are is not important for the action of the laser itself, although without them the laser would be impossible. Sometimes it is possible to maintain the inversion even while the laser is working; that is, it is possible to supply enough energy by the mechanisms that excite the atoms (inelastic collisions with other kinds of atoms, for example) to compensate for the energy emitted as radiation. These lasers can therefore operate continuously.

There are two reasons laser light is very desirable for certain applications. First, it can be extreme intense; some lasers can emit millions of joules in minute fractions of a second, as all their atoms emit their stored energy at once. Second, it is coherent; the light waves are all in step with each other. Incoherent light waves are somewhat like the waves crisscrossing the surface of a pond in a gale. But coherent waves are like those in a ripple tank, or at a beach where tall breakers arrive rhythmically.

The high intensity of some lasers can be used for applications in which a large amount of energy must be focused on a small spot. Such lasers are used in industries for cutting and welding delicate parts. In medicine, they are used, for example, to reattach the retina (essentially by searing a very small spot) in the eye.

The coherence of lasers is used in applications that require a stable light source emitting light of a precisely given frequency and polarization in one precise direction. Surveyors can use lasers to lay out straight lines, since the coherent beam spreads out very little with distance. Telephone companies can use them to carry signals in the same way they used radio and microwaves. And DIPC researchers can use them to illuminate materials to study their properties.


Asimov, I. (1993) New Guide To Science Penguin Press Science

Cassidy, D. et al (2002) Understanding Physics Springer Verlag New York

Author: César Tomé López is a science writer and the editor of Mapping Ignorance.

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