Gravitational Lensing

What does happen when light goes through a lens? It basically bends; and because of the bending of light we see an image of the object emitting light. From Einstein’s theory of general relativity we know that if there is enormous amount of mass, space-time curvature is bent there. Light should also bend there. Therefore we should observe exactly same thing as when we look through a lens. We should see a cosmic lens standing between us and distant galaxy.

This NASA Hubble Space Telescope photo reveals a cosmic kaleidoscope of a remote galaxy, which has been split into multiple images by an effect called gravitational lensing. Courtesy: Hubble Space Telescope

So, a gravitational lens is a distribution of matter, such as cluster of galaxies, between a distant light source and an observer, that is capable of bending light from the source as the light travels towards the observer (Attribution: Wikipedia). This effect is known as gravitational lensing.

Optical lenses produce maximum deflection of light that passes close to its edges and a minimum close to its center. A point-like gravitational lens, however, acts exactly opposite to this, produces maximum deflection of light that passes close to its center and minimum deflection of light that travels furthest from its center. This phenomenon has a significant consequence, that is, a gravitational lens has no single focal point, but a focal line. If the (light) source, massive lensing object and the observer lie in a straight line, the original light source will appear as a ring around the massive lensing  object and if there is any misalignment, the observer will see an arc segment instead.