MadSci Network: Physics

Re: Does light have to be accelerated to the speed of light or isit always trav

Date: Mon Apr 16 17:45:50 2001
Posted By: Ronald Fisch, Physics, Washington University
Area of science: Physics
ID: 986432890.Ph

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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