A gravitational lens occurs when the gravity of a massive object deflects the light from a distant source object situated behind it. This bending of light results from the warping of spacetime, as predicted by Einstein’s General Relativity {Einstein_1936}. In 1937, Fritz Zwicky proposed that massive objects, such as galaxies or clusters of galaxies, could act as gravitational lenses, bending the light from more distant objects behind them {Zwicky_1937}. He suggested that this effect could be used to detect and study unseen objects and measure the masses of galaxies. Zwicky’s ideas laid the groundwork for what would later become a critical tool in astrophysics for studying dark matter, galaxy formation, and the large-scale structure of the universe. Strong lenses can form an “Einstein ring” or an “Einstein cross” and are invaluable for magnifying light from faint background sources like galaxies and stars {Congdon_2018}. Figure 1 shows a cluster of galaxies producing several distinct arcs and distortions.

The first observationally confirmed gravitational lens phenomenon occurred in 1979/80 with the observation of the quasar QSO 0957+561 A/B, also known as the Twin Quasar {1980_Young}. This was the first confirmed large-scale galaxy gravitational lens. Earlier, in 1919, Arthur Eddington confirmed Einstein’s theory by observing the deflection of light during a solar eclipse, which provided strong evidence for General Relativity.

The Twin Quasar system is a quasar that appears as two images, resulting from lensing effects caused by the galaxy YGKOW-G1, which is situated between Earth and the quasar. Figure 2 shows an image from ESA/Hubble and NASA that depicts the Twin Quasar as two bright objects at the center. They appear bluish color and have diffraction spikes.