Bailyn, Charles. ASTR 160: Frontiers and Controversies in Astrophysics, Spring 2007. Yale OpenCourseWare: Astronomy, Accessed 22/9/14 http://oyc.yale.edu/astronomy/astr-160/lecture-14 License: Creative Commons Attribution-Noncommercial-Share Alike 3.0 license Professor Bailyn begins with a summary of the four post-Newtonian effects of general relativity that were introduced and explained last time: precession of the perihelion, the deflection of light, the gravitational redshift, and gravitational waves. The concept of gravitational lensing is discussed as predicted by Einstein's general relativity theory. The formation of a gravitational lens can be observed when light from a bright distant source bends around a massive object between the source (such as a quasar) and the observer. Professor Bailyn then offers a slideshow of gravitational lenses.