A: It is generally accepted that the speed of light in a vacuum is not dependent on its frequency (color). To understand why, we need to look at what happens when light passes through matter, where its speed does vary with frequency.

Dispersion is the variation of the speed of light with frequency of light. Newton was the first to really get an experimental handle on dispersion in glass to an extent that allowed him to build better refracting telescopes. But it wasn’t until the first half of the last century, with the advent of quantum mechanics, that we were able to really understand the origin of dispersion in the interaction of light with matter.

Since light is a transverse electromagnetic wave and matter is composed of tiny charged particles, light and matter will necessarily interact. When light propagates through some transparent medium, glass for example, the oscillating electric field forces the electrons to move back and forth at the same frequency. These oscillating electrons create their own electromagnetic waves, which, when added together, form a secondary wave that is 90°, or a quarter wave, behind the phase of the original wave. The original wave and the secondary waves add up, or interfere, to form a new wave that is a little behind the original wave; we’ll call this the aggregate wave. The more matter the light travels through, the more the aggregate wave is behind the original wave. This is one picture of how light is slowed down in normal matter. Using this picture we can explain how dispersion works.

The oscillating electrons are somewhat fixed in matter, almost as if they are attached to nuclei by springs. Like any spring system, they want to vibrate at a specific frequency called the resonant frequency. When forced, they will vibrate at other frequencies but their oscillations are not as strong. Consequently the more the frequency of light differs from the resonant frequency the weaker are the secondary waves, and the less the aggregate wave is slowed down. This interaction with matter and interference of secondary waves is the origin of dispersion and it requires the existence of matter. Thus, in a vacuum, or more precisely, free space, where there is no matter, the speed of light should not vary with frequency according to this picture. In general most physicists and engineers consider the speed of light to be fixed with respect to frequency in a vacuum.

However, within this picture it has been assumed that the photon, a particle of light, has zero mass. And while this is generally accepted as correct, it is really just a theoretical assumption and subject to verification. It turns out that if the photon has a non-zero mass then its speed in a vacuum would vary with frequency. This variation would be very small and of little or no concern to most modern engineers, but if it exists there are profound implications for our understanding of physics.

Daniel G. Smith

Follow these links for more information about the mass of a photon and the variation of the speed of light in a vacuum.

Experimental limits on the photon mass and cosmic magnetic vector potential

American Institute of Physics

A New Limit on Photon Mass

Photon Mass Gets a Boost

Severe Limits on Variations of the Speed of Light with Frequency

Particle Data Group, 2003 Review of Particle Physics, data on photon mass