RE: blue flash and criticality

["Tom Morgan" <tmorgan@isotopeproducts.com>]

Cerenkov light is produced when electrons travel faster than the phase
velocity of light in the medium through which they are traveling.  To
calculate this threshold velocity one need only divide the speed of light
(in vacuum) by the refractive index of the medium, c/n.

Vacuum, by definition, has a refractive index of 1.0000000.  Water, at 20 C,
has a refractive index of 1.33335 for sodium light (( = 0.5893 um).  Air has
a refractive index of 1.0002926 at STP (wave length not specified).

For electrons, this works out to about 240 keV in water and about 21 MeV in
air.