Vergence

The vergence of light is defined by

where n is the index of refraction of the medium and L is the distance in accordance with the Cartesian sign convention.

Since the distance L1 is measured from the wavefront and light is traveling left to right, it is a negative distance and the vergence is negative (divergent). L2 is positive since it is directed to the right from the wavefront (convergent).

The change in vergence when the light encounters a refracting surface is equal to the power of the surface:

Vergence Example

The example below shows a positive lens followed by a negative one, but the powers can be given any value, positive or negative. At each surface the relationship V + P = V' is applied, and the change in vergence between surfaces is calculated.