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.
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