Light = Electromagnetic Waves?

Posted on 2007-08-15 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2007_08_15.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on polarization waves, nonlinear polarization

It is very commonly assumed that light is physically nothing else than electromagnetic waves. One then easily overlooks a wide range of essential phenomena which are also involved in many cases of light propagation. This occurs as soon as light enters some medium:

• In a transparent medium, an electromagnetic wave will inevitably interact with all the charged particles. This interaction may be very weak for waves with very high frequency, such as X-rays or gamma rays. In the optical spectral region, however, the interaction substantially modifies the propagation characteristics. The usually most important effect is that the electromagnetic wave induces an electric polarization wave, which itself modifies the electromagnetic field. This interaction usually leads to a reduced phase velocity, as determined by the refractive index, and to chromatic dispersion, arising from the frequency dependence of such quantities. The fact that we can still describe such phenomena with Maxwell's equations, simply including some electric susceptibility of the medium, somewhat disguises that not only some electric and magnetic fields are interacting with each other.
• In some extreme cases, we obtain slow light, having a group velocity orders of magnitude below that of light in vacuum. What propagates there is far from a pure electromagnetic wave. In the most extreme experiments, a pulse of light is even brought to a halt. Its energy is effectively stored in the medium, until the pulse is released again by modifying some conditions.
• Another interesting phenomenon is that of plasmons, which are essentially electric oscillations involving the free carriers of some metallic particles or surfaces. One may be tempted not to use the term light for such phenomena, but it is not easy to set up definitions which clearly separate such phenomena into different domains.
• In the context of nonlinear frequency conversion, new frequency components are generated by radiating nonlinear polarization waves.

So we see that only light in vacuum can be fully described as an electromagnetic wave, while there is much more to be explored for light propagating in media.