|MadSci Network: Physics|
When a photon is emitted into vacuum, it is instantaneously traveling at the speed of light. Its zero mass is important. If it did not have zero mass, then some time would be required to accelerate it. But not all photons travel at "the speed of light". A photon in some material medium, such as water, travels more slowly than c, the speed a photon travels in vacuum. The speed at which a photon travels in the medium is c/n, where n is the index of refraction. Because n depends on wavelength, light of different colors travels at different speeds in the medium. This produces dispersion. If we have some excited atom inside a closed box, and the atom decays into its ground state by emitting a photon, then the photon will be trapped inside the box. This trapped photon is a standing wave, rather than a traveling wave. There are other ways of creating standing waves of photons inside some box. Many useful devices, such as the klystron and some types of lasers use standing waves of photons to create coherent electromagnetic radiation.
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