In empty space light propagates along straight lines. According to the General Theory of Relativity masses curve space-time, such that lightrays by following the shortest paths in this geometry, seem to become deflected.

Since the light of one and the same object arrives at the observer on different paths s/he may see deformed, enlarged, or multiple images of the observed object at the sky. Masses, who act this way similar to optical glass lenses are called gravitational lenses.

The effect becomes more pronounced, if the mass of the lens becomes larger. In astrophysics these days stars, black holes, galaxies, and galaxie clusters are known as lenses.