Yet the expansion of the universe causes a redshift, not a Doppler effect. Events at or near the speed of light are really some of the most fun and surprising of all physics phenomena. So we'll try to give some additional insight into Santa's relativistic world. This way of thinking, however, does not always lead to an easy understanding of all experimentally observed phenomena related to light. To easily understand the reduction of the speed of light inside matter, physicists take advantage of light's split personality.
Light can behave like a particle and like a wave.
Traveling through matter, light waves interact with the electrons of the material. The theoretical description is based on the famous Maxwell's equations. Knowing the electromagnetic properties of a material, such as the dielectric constant, scientists can compute the reduced speed of light, the speed at which light waves travel through matter. The Doppler effect, originally a theoretical prediction, was first observed for sound waves, in 1845, long before the sirens on police cars made this a publicly known phenomenon. It took another 60 years until the Doppler effect was also observed for light, in experiments with beams of light- emitting ions.