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 standard use of vergence expresses the distance L in meters, so the unit of vergence is m^{1}, often called "diopters".
The change in vergence when the light encounters a refracting surface is equal to the power of the surface P_{s}:

Index Lens concepts Thick lens concepts MeyerArendt Ch 1.1  

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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. This calculation will make use of the surface powers of the lenses, their indices of refraction and thickness, and the separation of the lenses. Note that the vertical displacements of the indicated light path are greatly exaggerated since the application of vergence tracing for thick lenses is limited to the small angles of paraxial rays. A default set of values for the parameters of this calculation is provided. You can see the default calculation by entering a 0 into one of the lens parameters, which will trigger the entry of the default parameters for the lenses. Any of them may be then changed to explore the behavior of the lens system.

Index Lens concepts Thick lens concepts  

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