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The line of sight, also known as visual axis or sightline , is an imaginary line between a viewer/observer/spectator's eye 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 - from 1mm to 1 mile away.
In optics, physical refraction from the use of lenses can cause distortion. Shadows, patterns and movement can also influence line of sight interpretation.
The term "line" typically presumes that the light 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, refracted by a lens or density changes in the traversed media, or deflected by a gravitational field.