Vergence

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The measure of the curvature (convergence or divergence) of a light wavefront at a specific position in space, defined as the reciprocal of the distance to its focal point, adjusted for the refractive index of the medium.

Quick Decode

Optical vergence describes how strongly a light wavefront is converging toward a point or diverging from one at a particular location. It is calculated as V = n/r, where n is the medium’s refractive index and r is the signed distance in metres to that point; vergence is measured in dioptres. Under the usual optical sign convention, converging light has positive vergence, diverging light has negative vergence, and parallel light has zero vergence.

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References (1)
  1. Keating, M. P. Geometric, Physical, and Visual Optics (2nd ed., Chapter 6).