Gravitational lensing is the focusing of light by large masses situated between an emitter and an observer. If there is azimuthal symmetry along the line of sight, then the observer would observe a point-source as a ring around the deflector. If the deflector is asymmetrical or the medium is inhomogeneous, a point source can appear as a number of distinct possibly distorted images. Chwolson (1924) and Eddington (1920ab) were the first to suggest gravitational lensing as a consequence of general relativity.
Deflection Angle, Einstein Cross, General Relativity
References
Chwolson, O. "Über eine mögliche Form fiktiver Doppelsterne." Astron. Nachr. 221, 329-330, 1924.
Eddington, A. S. Report on the Relativity Theory of Gravitation. London: Physical Society, 1920a.
Eddington, A. S. Space, Time, and Gravitation: An Outline of the General Relativity Theory. Cambridge, England: Cambridge University Press, 1920b. Reprinted by Cambridge University Press in 1987.
Einstein, A. "Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field." Science 84, 506-507, 1936.
Kochanek, C. S. and Hewitt, J. N. (Eds.). Astrophysical Applications of Gravitational Lensing. Dordrecht, Netherlands: Kluwer, 1996.
Perlick, V. "Gravitational Lensing." §8.4 in Ray Optics, Fermat's Principle, and Applications to General Relativity. Berlin: Springer-Verlag, pp. 199-210, 2000.
Schneider, P. P.; Ehlers, J.; and Falco, E. E. Gravitational Lenses. Berlin: Springer-Verlag, 1992.
Trimble, V. and Reisenegger, A. (Eds.). Clusters, Lensing, and the Future of the Universe. ASP Conference, Vol. 88.
Weisstein, E. W. "Books about Gravitational Lensing." http://www.ericweisstein.com/encyclopedias/books/GravitationalLensing.html.
Zwicky, F. "Nebulae as Gravitational Lenses." Phys. Rev. 51, 290, 1937.
