The line of sight, also known as visual axis or sightline (also sight line), is an

imaginary line In general, an imaginary line is usually any sort of geometric line (more generally, curves) that has only an abstract definition and does not physically exist. They are often used to properly identify places on a map. Some outside geograph ...

between a viewer/ observer/ spectator's eye(s) and a subject of interest, or their

relative direction In geometry, direction, also known as spatial direction or vector direction, is the common characteristic of all ray (geometry), rays which coincide when translation (geometry), translated to share a common endpoint; equivalently, it is the commo ...

. The subject may be any definable object taken note of or to be taken note of by the observer, at any distance more than least distance of distinct vision. In

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

, refraction of a ray due to use of

lenses A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

can cause

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

. Shadows, patterns and movement can also influence line of sight interpretation (as in optical illusions). The term "line" typically presumes that the

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

by which the observed object is seen travels as a straight ray, which is sometimes not the case as light can take a curved/angulated path when reflected from a

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...

ed by a

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

changes in the traversed media, or deflected by a gravitational field. Fields of study feature specific targets, such as vessels in navigation, marker flags or natural features in surveying, celestial objects in astronomy, and so on. To have optimal observational outcome, it is preferable to have a completely unobstructed sightline.

Applications

Sightline (architecture) In architecture and urban planning, sightlines or vistas are a consideration in the design of civic structures, such as a stage (theatre), stage, arena, or monument. They may determine the configuration of architectural elements in theater (st ...

* Line-of-sight fire, shooting directly at a visible target on a relatively flat trajectory. The target is in the ''line of sight'' of the sighting device and the rifleman's rule applies. * Line-of-sight range * Line-of-sight (missile), the straight line between the missile and the target *

Radar horizon The radar horizon is a critical area of performance for air traffic, aircraft detection systems, defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at the lowest level possible. I ...

Line-of-sight propagation Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves can only travel in a direct visual path from the source to the receiver without obstacles. Electromagnetic transmission in ...

, electro-magnetic waves travelling in a straight line **

Non-line-of-sight propagation Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections. Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physi ...

Line-of-sight velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

, an object's speed straight towards or away from an observer * Line-of-sight

double star In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a ...

, one in which two stars are only coincidentally close together as seen from Earth *

Beyond visual line of sight An unmanned aerial vehicle (UAV) or unmanned aircraft system (UAS), commonly known as a drone, is an aircraft with no human pilot, crew, or passengers onboard, but rather is controlled remotely or is autonomous.De Gruyter Handbook of Drone ...

References

{{wikt Geometry Line (geometry) Vision