Distance to the Quasar
The lensing effect observed for the quasar in the left panel has an important side benefit.
In order to bend the light from the quasar,
the galaxy causing the lensing must be closer to us than the quasar. It appears to be a giant elliptical galaxy
and so is of high luminosity, but it is very faint in the double quasar image. This implies that it must be
very far away, which proves that the quasar is very distant too.