Every good book about electromagnetic theory works out the Fresnel
Equations. They are based on the boundary conditions on electromagnetic
waves propagating from one medium into another. For transparent media
there has to be a transmitted wave and a reflected wave. The phase
change that you are talking about is mandated by the boundary conditions
on the fields (electric and magnetic) at the interface. Hope this
helps! WBN
Barlow Newbolt
Professor of Physics
Washington and Lee University
e-mail: newboltw@wlu.edu
telephone: 540-463-8881
fax: 540-463-8884
Office: Howe 218
"Had I been present at the creation, I would have given
some useful hints for the better ordering of the universe."