Optical Properties and Topological Configuration of Caustics of Micro Gravitational Lenses

The mathematical properties of gravitational lens equations are examined in the frame work of gravitational micro-lensing effects. The caustics of the gravitational lens may be defined in terms of "cusps" and "folding" in general. In some cases for over-focusing, however, the critical curves(caustics) have no cusp and no folding. If the observer is in the over-focussed region, he may not see any lensed image.