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 that has only an abstract definition and does not physically exist. In fact, they are used to properly identify places on a map. Some outside geography do exist, such as th ...

between a viewer/ observer/

spectator ''Spectator'' or ''The Spectator'' may refer to: *Spectator sport, a sport that is characterized by the presence of spectators, or watchers, at its matches *Audience Publications Canada * ''The Hamilton Spectator'', a Hamilton, Ontario, Canada, ...

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

(s) and a subject of interest, or their relative direction. 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 instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

, refraction of a ray due to use of

lenses A lens is a transmissive optical device which 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. Shadows, patterns and movement can also influence line of sight interpretation (as in

optical illusions Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...

). The term "line" typically presumes that the

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

by which the observed object is seen travels as a straight

ray Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (g ...

, which is sometimes not the case as light can take a curved/angulated path when reflected from a

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

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 phenomeno ...

ed by a lens or

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

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) *

Line-of-sight range In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals t ...

Line-of-sight (missile) In missile guidance, line of sight (LOS) is the line directly between the launcher/tacker and the target, which must be in view. If the target is moving the missile is constantly manoeuvred to be on the line of sight as it changes, this results in t ...

, the straight line between the missile and the target *

Line-of-sight propagation Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions travelin ...

, electro-magnetic waves travelling in a straight line ** Non-line-of-sight propagation *

Line-of-sight fire Direct fire or line-of-sight fire refers to firing of a ranged weapon whose projectile is launched directly at a target within the line-of-sight of the user. The firing weapon must have a sighting device and an unobstructed view to the target, w ...

, shooting directly at a visible target on a relatively flat trajectory *

Line-of-sight velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection of ...

, 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 bi ...

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

Beyond visual line of sight The use of unmanned aerial vehicles (UAVs) or Drones, is generally regulated by the civil aviation authority of the country. Nevertheless, the International Civil Aviation Organization, International Civil Aviation Organization (ICAO) began explorin ...

References

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