Because the sum of the interior angles of a triangle is 180 deg, we see that the apex angle for both of the isosceles triangles is (180 - 2*B). Since the sum of all the angles at the center of the circle is 360 deg we get the following relationship:
360 = A + (180 -2*B) + (180 - 2*B) + D
D = 4*B - A
But if we look at the exit beam we see there is another relation ship for D, D = G + A and so we end up with the final angle we really want:
G = D - A
G = 4*B - 2*A

But we can use the relationships above, namely
sin(A) = H
A = sin^-1( H )
and
n * sin(B) = sin(A) = H
B = sin^-1( H / n)
continue
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