The near object's gravity bends the light rays from the more distant object and magnifies them like a lens.

BBC: Artist's concept of a distant world (BBC)

Because gravity pulls on light as well as matter, when light passes by a heavy object it bends as though it is passing through a lens.

ECONOMIST: Cosmology

Perhaps its greatest moment came in 1979 when it was the first telescope to observe a gravitational lens, an effect where a "massive" object bends electromagnetic radiation such as light around it.

BBC: Aye to the telescope

The lens in glasses brings the focus closer and makes the object you are looking at appear bigger.

BBC: Eyesight: The effect of age