Re: [ATM] What is the "dielectric" in dielectric coatings?

[Mike Lockwood <melockwo@uiuc.edu>]

Vladimir,

Vladimir Galogaza wrote:
> I know that dielectric coatings can decrease reflection from lenses, but
> I see no way to increase reflection of metal mirrors ( Al coated glass
> substrate
> for example) by dielectric coatings of Al.
> I am open to good news though.
> (Preferably with explanation of involved physics.)

See this page for a good discussion of dielectric coatings:
   http://www.mellesgriot.com/products/optics/oc_5.htm

The selection of the thicknesses of the layers of the coatings allows 
the light reflected from various layers to stay in phase.  The 
simplest example of this is a 1/2 wave-thick layer, adjusted for 
desired angle of incidence, say 45 degrees for an enhanced secondary 
mirror, and adjusted for the speed of light in the particular 
material.  The light travels through the layer twice, for a total of 1 
wave of path length difference.  So the layer contributes a wavefront 
that is in phase with the wavefront reflected by the other layer(s).

It can be thought of in the following sense: much of the light is 
reflected by the top layers, but the light that isn't goes through and 
hits another layer below it, and most of that light is reflected by 
it.  That secondary reflected wavefront is in phase with the first 
one, so they add constructively and the light is not lost.

The two wavefronts may be out of phase by a whole number of waves, but 
we will never notice.

A nice explanation of single and multi-layer antireflective coatings 
can be found at
   http://www.mellesgriot.com/products/optics/oc_2_2.htm

They are the opposite - coatings or many layers of coatings are made 
the proper thickness such that the unwanted reflections destructively 
interfere, and are reduced to very small amounts.

	Mike Lockwood

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