If tangents from two points E and D are drawn to a conic with foci A and B, and these tangents respectively intersect at F and G as shown below, then ∠EFD + ∠EGD = ∠EAD + ∠EBD [or as shown by the dynamic measurements on the bottom left α + β (= γ) = δ + ε (= ζ)].
This beautiful theorem crystalized from experimental observations in 1986 by Dick Nickalls (retired) from the Dept. of Anaesthesia, Nottingham University Hospitals, UK in the area of vision physiology - a rich source of mathematical problems. It relates to the so-called Pulfrich effect, an optical illusion in which an object which is moving in a plane parallel to the viewer's forehead seems to move out of that plane and to approach (or recede from) the viewer.
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Drag points A, B, C, D or E and observe the theorem dynamically. Also drag the ellipse until it becomes a circle. What do you notice?
The theorem is also useful in providing easy proofs for other fundamental properties of conics such as "The circumcircle of the triangle formed by any three tangents to a parabola passes through the focus" (e.g. see Parabola Circumcircle Theorem), "The locus of the foot of the perpendicular from the focus of a parabola to a tangent is the tangent at the apex" (e.g. see Parabola Locus Theorem), etc.
Michael de Villiers, updated 16 March 2012, created with GeoGebra