lensing

Lensing is the bending or distortion of light by a foreground object or medium, altering the apparent position, shape, or brightness of background sources. It occurs in two broad contexts: optical lensing, produced by physical lenses that refract light; and gravitational lensing, produced by mass bending spacetime.

Optical lensing uses transparent materials with curved surfaces, such as camera lenses. Refraction changes the light’s direction, producing convergence or divergence. The thin lens equation relates object distance, image distance, and focal length. Real images form when rays meet; virtual images form otherwise. Lenses can exhibit aberrations that limit image quality.

Gravitational lensing arises from general relativity: mass curves spacetime and light follows these curves. In strong lensing, background sources produce multiple images, arcs, or Einstein rings. In weak lensing, subtle distortions of many sources map the foreground mass; microlensing involves temporary brightening as a compact object transits the line of sight.

Applications include mapping dark matter, measuring cosmological parameters, and detecting distant galaxies and exoplanets. Gravitational lensing also tests gravity on large scales. Challenges include modeling degeneracies and the need for high-resolution data.