VergenceThe 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.
The change in vergence when the light encounters a refracting surface is equal to the power of the surface P_{s}:

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Vergence ExampleThe 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_{s} = V' is applied, and the change in vergence between surfaces is calculated. 
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