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    From: Bill Arden
    Date: 2004 Apr 23, 16:20 EDT
    Hi, all -
    Just my $0.02...
    I believe that, at non-normal incidence angles, a ray reflected from a plane-parallel piece of glass DOES result in two reflected rays, displaced from each other.
    The ray from the front surface reflects at an angle equal to the angle of incidence (about 4%, as you've discussed).
    The rest of that ray is refracted into the glass at an angle given by Snell's Law, reflects off the back surface at that angle, then is refracted again on its way out of the front surface. It emerges parallel to the directly reflected ray, but displaced from it.
    If I have my geometry / trigonometry right, the perpendicular distance between the two emerging rays is (2t/n) * sin(theta) * cos(theta), where t is the thickness of the glass, n the refractive index of the glass (I'm assuming the index of air is 1), and theta is the angle of incidence (measured from the normal) of the incident ray.
    Isn't that true?
    Bill Arden
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