Divergence (optics)
The term divergence is used in two different meanings in geometric optics :
Divergence of a beam path
When designing an optical imaging system, a distinction is made between areas in which the light (more precisely the bundle of rays ) runs parallel , convergent (towards a center) or divergent (away from a center).
A collecting lens ( collimator ) that collects sunlight at its focal point is the simplest example of such a beam path: the sun's rays run parallel in front of the lens, convergent towards the focal point after the lens and divergent apart after the focal point.
Divergence of a radiation source
To characterize the radiation characteristics, the divergence is a measure of that opening angle of the beam path, which is also unavoidable in the parallel bundle because every real light source is a surface emitter . An ideal point light source in the focus of a converging lens (or at an infinite distance) would actually deliver a parallel bundle of rays within the scope of geometric optics (i.e. neglecting light diffraction ) - this cannot be achieved with a real radiation source.
This has the consequence that, for example, the above-mentioned focal point, which a converging lens produces with sunlight, has a finite extent even without the effect of light diffraction, that is to say is not an ideal point.