LS: Speaking of Science Column February 02, 2009

Protons stream from atoms as intense beam

Previously we discussed how atoms only can exist in specific energy states and how adding energy to an atom requires that light have specific colors.

Light of a single color (monochromatic light) is composed of photons that all have the same energy.

Just as an atom can absorb energy from the right-size photon, it can “dispose” of its extra energy by emitting a photon having an energy equal to the spacing between its energy levels.

Because the spacings between the energy states are unique to the type of atom, it emits colors specific to that type of atom. The emission of photons by excited atoms in this manner is “spontaneous,” meaning that nothing else has to be done to the atom to make it happen.
In addition to recognizing that light can be described as a beam of particles, Albert Einstein predicted that an excited atom could be induced to emit a photon if another photon having exactly the right energy (equal to the spacing of the atom’s energy states) passed by the excited atom.

In this case the photon passing by “stimulates” the emission of the second photon. The important difference between the spontaneous emission of a light photon and the stimulated emission of a photon is that in the latter case the two emitted photons are identical. They have the same energy and travel in the same direction.

Sometimes it is more convenient to think of light as a wave rather than a particle (we are allowed to do this because light has the properties of both waves and particles). The photons from stimulated emission have the same phase, meaning their waves have their peaks and valleys at exactly the same time. They are said to be “coherent.”

To cohere means to stick together. Coherent photons travel in exactly the same direction, forming the narrow beam of light we see from a laser. Two excited atoms could spontaneously and simultaneously each emit a photon having the same energy, but these would leave in independent directions and not be coherent with each other.

To stimulate the emission from one excited atom is scientifically interesting but not very “gee whiz.” To make a laser, we have to have a lot of excited atoms at the same time. With many atoms excited at the same time, we can produce a chain reaction through stimulated
emission. Initially, one spontaneously emitted photon shoots out from an atom and strikes another excited atom, stimulating it to emit an identical photon; these photons go on to stimulate the emission of more photons, and so on.  

Because the stimulated photons from a laser are coherent, travelling in the same direction and having the same energy (color), we produce an intense, monochromatic, narrow beam of light energy. If the laser is shaped like a rod or a tube, it is more likely that the beam will come out along the axis of the tube. We also may add mirrors at the ends of the rod or tube so that the desired beam feeds back and forth on itself, stimulating more emission and building up its intensity.

One of the mirrors is made partially transmissive to let the light energy leak out.

Returning to our simple analogy described in the first part of this series, a monkey spontaneously drops from a tree and runs over to another tree, shakes it, and a second monkey drops down. The two monkeys then run on to two more trees and shake them, producing four running monkeys. These go on to more trees until we have a thundering herd of monkeys, all running in exactly the same direction in lock step.


Jack Kingsley received a doctorate degree in physics from the University of Illinois. He spent his career in research and development with General Electric Co. in Schenectady, N.Y.

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