The concept of divergence is used in the geometrical optics in two different meanings:
Divergence of a beam path
In the design of an optical imaging system to distinguish areas in which the light (specifically, the beam ) in parallel, convergently (on a center to ) or divergent (from the center) extends.
A condenser lens which collects sunlight to its focal point, the simplest example of such an optical path, the sun rays pass in front of the lens in parallel, after the lens is convergent on the focal point on and after the focus divergent apart.
Divergence of a radiation source
For characterization of the radiation pattern, the divergence is a measure of opening angle to those of the beam path, which is inevitable in the parallel beam, because each real light source is a surface emitter. An ideal point light source in the focus of a converging lens (or at infinity ) would be in the scope of geometrical optics ( ie neglecting the diffraction of light ) actually provide a parallel beam - at an actual radiation source, this can not be achieved.
This has the consequence that, for example, the above-mentioned focal spot which is produced by a convergent lens with sunlight, even without the effect of the diffraction of light has a finite size, so is not an ideal point.