Gravitational lensing: Of catastrophes and wave effects

Gravitational lensing has now been known as a physical phenomenon for 101 years. In the last five decades, gravitational lensing has progressed from an exotic phenomenon to something that can be used routinely as a tool in astrophysics.  I will discuss two extreme instances of gravitational lensing.  Strong gravitational lensing by galaxies and clusters of galaxies has been used for a variety of applications but the focus has mostly been on stable image formations of fold and cusp. As we progress towards deep surveys in multiple wavelengths the number of known lens systems is expected to increase by several orders of magnitude. This will naturally result in observations of rarer and unstable image formations. I will describe a catastrophe theory based approach for locating singularities in lens maps corresponding to these image formations. I will also discuss the likelihood of discovering such image formations in upcoming surveys and potential applications of the same. The second aspect of gravitational lensing that I will describe are wave effects. Lensing of gravitational waves, especially microlensing requires incorporation of wave effects as the eikonal approximation is not valid. I will discuss implications of this for observations of gravitational wave sources.