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Light rays bend in the presence of a gravitational field.
The bending of light in a gravitational field was a prediction of Albert Einstein in 1912, even before he published his General theory of relativity.
A gravitational lens is a distribution of matter in between a distant light source and an observer. An example of this kind of distribution of matter could be a cluster of galaxies. This distribution of matter is capable of bending the light from the source as it travels toward the observer.
Interestingly enough, the classical physics that existed prior to general relativity predicted the bending of light, however, it was only about half of that predicted by General relativity.
Gravitational lenses work equally well on all kinds of electromagnetic radiation, not just visible light.
In a gravitational lens:
maximum light deflection is produced closer to the center
minimum light deflection is produced closer to the edge
Thus, the gravitational lens does not have a focal point, however, a focal line.
An Einstein ring, is created when light from a star or galaxy passes by a massive object on its way to the Earth. If the light source, the gravitational lens and the observer are all in alignment, then it appears as a ring.
What is more common, however, is that the lensing be more complex. This does not cause a spherical distortion of spacetime. The source resembles partial arcs scrambled around the lens. There may even be multiple images of the same source.
There are 3 kinds of gravitational lensing:
1. Strong gravitational lensing - These are easily visible distortions.
2. Weak gravitational lensing - These distortions are much smaller. These distortions can only be detected by analysis of large numbers of sources.
3. Gravitational microlensing - No distortion of shape can be seen here. The amount of light received, changes with time.
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