Re: mpb-discuss Digest, Vol 15, Issue 11

On Fri, 25 May 2007, Raman S wrote:
> There is a sqrt(epsilon1)+sqrt(epsilon2) there, how
> does that arise? How does that expression boil down to
> the quater wave?

sqrt(epsilon) is the index of refraction.  In a quarter-wave stack, the 
two layers are chosen to have the same optical thickness (the same phase 
difference), and therefore the thicknesses d1 and d2 must satisfy:

 	n1 d1 = n2 d2

Therefore, if we require d1 + d2 = a, we obtain:

 	d1 = a * n2 / (n1 + n2)

 	d2 = a * n1 / (n1 + n2)

which is the condition in the .ctl file that you quote.

See e.g. _Optical Waves in Layered Media_ by Yeh for more information on 
this sort of thing.

Cordially,
Steven G. Johnson

PS. The above formulas are designed for normal incidence only.