When the constant surface mass density is above some critical value in this phenomenon, the source can be imaged as a circularly symmetric ring with an angular radius known as the Einstein radius. Refsdal showed how this phenomenon, applied to a variable source, could be used to calculate the Hubble Constant and giant blue 
When the constant surface mass density is above some critical value in this phenomenon, the source can be imaged as a circularly symmetric ring with an angular radius known as the Einstein radius. Refsdal showed how this phenomenon, applied to a variable source, could be used to calculate the Hubble Constant and giant blue luminous arcs and smaller arclets are the result of distortions generated by it. The micro regime results in a single brighter image, while the weak regime results in shear and convergence of the image. For 10 points, what is this phenomenon, the strong type of which creates multiple images when the light from a source such as a quasar is deflected due to the curvature of spacetime by a massive object such as a galaxy cluster?
When the constant surface mass density is above some critical value in this phenomenon, the source can be imaged as a circularly symmetric ring with an angular radius known as the Einstein radius.
{gravitational} {lens}ing