A bent path
Mass curves spacetime. Light follows that geometry, so a distant source can appear displaced from its unlensed position.
Gravitational lensA qualitative ray experiment
Light takes
the long way.
Move a dark lens through a field of distant light. Watch straight paths give way to arcs.
Drag the lens, or focus the sky and use the arrow keys.
The source stays distant.
The path changes here.
Lens position
More than one image
Different light paths can carry different images of the same source. Near alignment, extended arcs or a ring can appear.
Play with alignment
Move the lens across the scene and change its strength. This artwork illustrates the idea; it is not a relativistic ray tracer.