If this bundle hits a converging lens and the starting point is at the focal point of the lens, the vergence is immediately in front of the lens
The change in vergence, from to , is the power D of the lens. It is positive for converging lenses, negative for diverging lenses and is also given in diopters.
Conversely, if a plane wave ( , parallel rays) hits a converging lens, the bundle receives the positive refractive power D of the lens as vergence: immediately behind the lens applies
Positive vergence means convergence: the rays of the bundle run towards a focus. That is at a distance behind the lens.
As the focus approaches, the vergence increases faster and faster. The concept of vergence only makes sense in dimensions far above the wavelength . At a focus, vergence has a mathematical pole , that is, it diverges and changes its sign : Towards the focal point it would be positive infinite, then negative infinite. With the Gaussian beam of wave optics, on the other hand, the curvature of the wave fronts initially increases with vergence, but becomes smaller again close to the focus. As it passes through the focus, the curvature changes steadily from positive to negative, i.e. H. The wave fronts are flat directly at the intersection of the rays of geometric optics . Sufficiently far behind the focus, the vergence again coincides with the curvature of the wave fronts (both negative).