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The bending of light by a lens or by the atmosphere is an example of refraction, and is due to the change of the speed of light as it moves from one medium to another, such as from air to glass, or through air which is gradually changing in density. This behavior is an

example of "the principle of least action," or in the special case of light, "the principle of least time," which means that the path taken by a ray of light is the path that can be traversed in the least time.

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Image: Brian Clegg

Imagine you are a life guard on a beach, and see a swimmer drowning off shore. Since you want to get to the swimmer as quickly as possible, and you can run faster on sand than you can swim, you don't take a straight line path, but a bent one that gets you there in the least possible time. Likewise, light travelling from one medium to another bends, and the travel time from one point to another is minimized.

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The same general principle can be used to show that light follows the curvature of space, so light rays bend when passing by a massive object. By using the "gravitational telescope" provided by gravitational lensing, astronomers can study many phenomena such as the distribution of dark matter in galaxy clusters, and the nature of accretion disks around black holes.

Image: NASA/CXC/M.Weiss

For simple descriptions of gravitational lensing see:

For a somewhat more detailed discussion, with links to technical articles, see:

A video of a lecture on gravitational lensing:

Image: Robert Horton

Image: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

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"Here, There, & Everywhere" (HTE) is supported by the National Aeronautics and Space Administration under grant NNX11AH28G issued through the Science Mission Directorate.

HTE was developed by the Chandra X-ray Center, at the Smithsonian Astrophysical Observatory, in Cambridge, MA.

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