The speed of light, or of electromagnetic waves in a material medium is
given by 
1/
v
µ ε
=
where m is the permeability of the medium and e its permittivity 6 The spectrum of electromagnetic waves stretches, in principle, over an
infinite range of wavelengths Different regions are known by different
names; g-rays, X-rays, ultraviolet rays, visible rays, infrared rays,
microwaves and radio waves in order of increasing wavelength  from 10–2 Å
or 10–12 m to 106 m