Transcript: Purely by chance nature has created several hundred little optics experiments with gravity. This occurs whenever a single galaxy or cluster of galaxies lies directly along the sight line to a more distant object like a quasar, an active galaxy, or another galaxy. Gravitational optics is directly analogous to optics with light. The radiation can be bent, focused, magnified, or demagnified. Gravitational lensing always creates an odd number of images, but the central image is often demagnified and superimposed directly on the lensing galaxy. So astronomers typically see double images or quadruple images although a single eleven image lens has been seen. Magnification by lensing can help us see very distant objects in the universe, active galaxies or galaxies that would not be visible in any other way, and in addition to measuring the dark matter content of a galaxy, lensing can be used to measure the distance scale directly by the light travel time delay between the path of light taking two different routes around a galaxy.