Then the rays PF and PF' make the same angle with the tangent to C at P . Thus any light ray originating at F and reflected in the ellipse will also go through F' . Let T be any line tangent to C . The product of the distances from F and F' to T is constant, and equals b . If we constrain P to lie in D and P' to lie in D' , then P'' describes an ellipse.
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