In 1979, gravitational lensing became an observational science when the double quasar Q0957+561 was discovered by Dennis Walsh. A double quasar is a quasar whose image appears twice, because it goes through another heavenly body and is lensed. This lensing is caused by the force of gravity on light and by the spacetime curvature.

For an image of multiple quasars to be considered a lensed quasar and not merely two quasars that appear next to each other they must satisfy 5 basic criteria

These can be used to gauge values for the Hubble constant and the cosmological constant. They also can help finding dark matter or MACHOs.