added the fresnel mirror, give a definition of the coherence

double refraction explanation

added cases like lambda/4
stress polarimetry

the new discussion should be more clear.

and removed the recap (previously 7.1) to be consistent
with Schwanenberger.

and worked on the ac_circuit, added general solution
to first order linear ODE

explicit integration over charged sphere to demonstrate
the result obtained with Gauss law.

tbd:
 * make the differentials everywhere non-slanted
 * add the atmospheric electricity part

mainly collection of points, need to be detailed out,
find more references.
should add  references to the book in general

stack overflow

some things are still missing

wellenoptik (interferometer etc.o9

Genaues Lesen wäre noch gut. Und dann posten.

and i need to brush up the whole presentation

Minor update

Why this change is needed:

What this change accomplishes:

needs brush-up then merge!

including electrical dipole. should be finished by Tuesday...