Gravitational
Lensing
One important consequence of the influence of Einstein's gravitation on light is that gravitational masses can alter the direction of light and cause lensing effects.

Lensing of Galaxies

The following Hubble Space Telescope image shows a spectacular example of such a gravitational lens. The arc-like pattern spread across the picture like a spider web is an illusion caused by the gravitational field of the cluster. The arcs are actually distorted images of very distant galaxies that are being imaged by the gravitational lens: The cluster is so massive and compact that light rays passing through it are deflected by its enormous gravitational field, much as an optical lens bends light to form an image. The figure also exhibits multiple imaging, which is less common and happens when the distortion is large enough to produce more than one image of the same galaxy.

The following figure shows a gallery of images giving evidence for possible gravitational lensing (Ref).
 

 

Gravitational Lensing of Quasars

Gravitational lensing was actually discovered for quasars before it was found for galaxies. In 1979 two quasars only 6 seconds of arc apart were discovered and they were found to have identical redshifts and spectra. The probability of this happening by accident is exceedingly small, and it was postulated that this pair actually corresponds to the same quasar that is lensed so strongly by intervening mass that its image appears double as we view it from Earth. The adjacent image shows the "double quasar" QSO 0957+561. The two bright blue objects in the center are actually gravitationally lensed images of the same quasar.

This interpretation was bolstered by the discovery of a galaxy nearly in front of one of the images and a surrounding cluster of galaxies that are candidates for the intervening mass causing the lens effect. For a more detailed discussion of the double quasar and this image, see this discussion by Bill Keel.
 

The Einstein Cross

Here is another example of a gravitational lens called the Einstein Cross. In this image, a single object appears as four objects. A very distant quasar is thought to be positioned behind a massive galaxy. The gravitational effect of the galaxy has created multiple images through gravitational lensing on the light from the quasar. The stars in the foreground galaxy also seem to be acting as gravitational lenses, causing the images to change their relative brightnesses in these two photographs taken three years apart.

Some References for Gravitational Lensing

See also the discussion of gravitational lensing in the search for dark matter.