Gravitational lensing is a phenomenon caused by the curvature of spacetime around massive objects, which bends the path of light and other forms of electromagnetic radiation.

Gravitational lensing can be caused by any massive object, including stars, planets, black holes, and even galaxy clusters.

Gravitational lensing is typically divided into two categories: strong lensing and weak lensing. Strong lensing occurs when a massive object bends light so much that it creates multiple images of a distant object. Weak lensing occurs when the bending of light is too small to create multiple images, but it can still be detected by measuring the distortion of the shape of distant objects.

The Einstein radius is the radius of the region around a massive object where gravitational lensing occurs. It is defined as the distance from the lens to the point where the bending of light is strong enough to create a complete ring of light around the lens.

Gravitational lensing has a wide range of applications in astronomy, including measuring the mass of distant objects, studying the structure of galaxies and galaxy clusters, detecting dark matter, and probing the early universe.

A gravitational mirage is a phenomenon that occurs when light from a distant object is bent by the gravitational field of a massive object, causing the object to appear distorted or elongated.

Albert Einstein first predicted the existence of gravitational lensing in 1915, as a consequence of his theory of general relativity.

The first gravitational lens ever discovered was the Double Quasar, which was identified in 1979 by astronomers Dennis Walsh, Bob Carswell, and Ray Weymann.

Abell 2218 is the most massive gravitational lens known to date, with a mass of approximately 10^15 solar masses.

The Hubble Space Telescope was used to detect the most distant galaxy ever observed, GN-z11, through the gravitational lensing effect of a massive galaxy cluster.

The Event Horizon Telescope was used to measure the mass of the black hole at the center of our galaxy, Sagittarius A*, through the gravitational lensing effect of the surrounding stars.

OGLE-2003-BLG-235 was the first gravitational lens that was used to detect an exoplanet through microlensing.

OGLE-2006-BLG-109 was the first gravitational lens that was used to detect a binary exoplanet through microlensing.

OGLE-2011-BLG-0262 was the first gravitational lens that was used to detect a free-floating planet through microlensing.

OGLE-2016-BLG-1190 was the first gravitational lens that was used to detect a circumbinary planet through microlensing.