🚀 go-pugleaf

In article <1992Sep25.073432.29995@hplb.hpl.hp.com> davids@spira.bri.hp.com (david sixsmith (contractor)) writes:
>1. What is (are) the inherent qualitie(s) of a substance, such as glass,
>perspex, diamond, & water, which make it transparent ?

	Well, at the microscopic level it is the fact that the scattering
in the material is both elastic and coherent.  In a metal, some
induced electron flow can cause the scattering to be inelastic,
and such phenomena as fluorescence are always incoherent.
At the larger-scale level, it is also important that the
medium not have irregularities (frosted glass isn't as transparent as
clear glass, by most tests).

>2. What is the mechanism which makes the speed of light lower when travelling
>in such a medium as opposed to traveling in a vacuum ?

	Entrainment of charge.  Free (or nearly free) charges are
moving inside the medium in response to the electric excitation,
so that the energy in the wave is not entirely field energy,
but includes kinetic energy and binding energy of the charges,
so that the wave equation is significantly altered.
	One could (equivalently) note that excitations of the
charge give rise to a phase-shifted response wave, and the sum
of the incident wave and the response is a slower-moving
composite wave.

	John Whitmore