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Geometrical relationship between the wavefront, W(r), the angle τ between a ray and the z axis, and the distance d from the wavefront to the focus point. This diagram is rotated 90° from Fig. 1 to show the wavefront as a mathematical function W(r) of the distance r along the radial r axis drawn perpendicular to the optical z axis. The dashed line is tangent to the wavefront and perpendicular to the ray at the radial location r
