THE LASER AND STANDING LIGHT WAVES

The laser, the device that is at the heart of your CD player and fiber optics communications, provides a common example of a standing light wave. In most cases a laser consists of two parallel mirrors with standing light waves trapped between the mirrors as illustrated in Figure (1). The light comes from radiation emitted by excited atoms that are located within the standing wave.
How the light radiated by the excited atoms ends up in a standing wave is a story in itself. An atom excited to a high energy level can drop down to a lower level by emitting a photon whose energy is the difference in energy of the two levels. This photon will have the wavelength of the spectral line associated with those two levels.

Spectral lines are not absolutely sharp. For example, due to the Doppler effect, thermal motion slightly shifts the wavelength of the emitted radiation. If the atom is moving toward you when it radiates, the wavelength is shifted slightly towards the blue. If moving away, the shift is toward the red. In addition the photons are radiated in all directions, and waves from different photons have different phases. Even in a sharp spectral line the light is a jumble of directions and phases, giving what is called incoherent light.
In contrast the light in a laser beam travels in one direction, the phases of the waves are lined up and there is almost no spread in photon energies. This is the beam of coherent light which made it so easy for us to study interference effects like those we saw in the two slit and multiple slit diffraction patterns. These patterns would be much more difficult to observe if we had to use incoherent light.

Figure 1: Laser consisting of two parallel mirrors with standing light waves trapped between the mirrors.

The purity of the light in a laser beam depends upon the standing light wave pattern created by the two mirrors, and upon a quantum mechanical effect discovered by Einstein in 1915.
Einstein found that there were two distinct ways an excited atom could radiate light, either by spontaneous emission or stimulated emission. An example of spontaneous emission is when an excited atom is all by itself and eventually drops down to a lower energy level. The emitted photon can come out in any direction and can be Doppler shifted.
If, however, a photon with the right energy passes by the excited atom, there is some chance that the atom will emit a photon exactly like the one passing by. This is called stimulated emission. (The energy of the passing photon has to be close to the energy the atom would naturally radiate.)
It is the process of stimulated emission that can lead to a laser beam. Suppose we have a gas of excited atoms located between parallel mirrors. At first the atoms radiate spontaneously in all directions. (We assume that there is some mechanism to excite the atoms). After a while one of the photons hits a mirror straight on and starts reflecting back and forth between the parallel mirrors. As the photon moves back and forth, it passes by an excited atom, stimulating that atom to emit an identical photon.
Now there are two identical photons bouncing back and forth. Each is likely to stimulate another atom to emit an identical photon, and we have four identical photons, etc. Soon there are so many identical photons moving through the excited atoms that there is little chance that an atom can radiate spontaneously. All the radiation is stimulated and all the photons are identical to the one that started bouncing back and forth between the mirrors.
The mirrors on the ends of the laser are not perfect reflectors, a few percent of the photons striking the mirror pass through, forming the beam produced by the laser. The photons lost to the laser beam are continually replaced by new identical photons being emitted by stimulated emission. One of the tricky technical parts of constructing a laser is to maintain a continuous supply of excited atoms. There are various ways of doing this that we need not discuss here.