Editing Total internal reflection (section)

{{Short description|Complete reflection of a wave}}
{{Use dmy dates|date=December 2021}}
[[File:Aquarium total internal reflection.jpg|thumb|upright|'''Fig.{{nnbsp}}1''':{{big|&nbsp;}}Underwater plants in a fish tank, and their [[mirror image|inverted image]]s (top) formed by total internal reflection in the water–air surface<!-- The lamp illuminating the scene is just above the water near the left edge of the picture, but is not directly visible, because their is no refracted line of sight from the lamp to the viewing position. -->]]

In [[physics]], '''total internal reflection''' ('''TIR''') is the phenomenon in which [[wave]]s arriving at the [[interface (matter)|interface]] (boundary) from one [[Transmission medium|medium]] to another (e.g., from water to air) are not [[refraction|refracted]] into the second ("external") medium, but completely [[reflection (physics)|reflected]] back into the first ("internal") medium. It occurs when the second medium has a higher wave speed (i.e., lower [[refractive index]]) than the first, and the waves are incident at a sufficiently oblique angle<!-- "critical angle" is introduced below, after "rays" --> on the interface. For example, the water-to-air surface in a typical fish tank, when viewed obliquely from below, reflects the underwater scene like a [[mirror]] with no loss of brightness (Fig.{{nnbsp}}1).

TIR occurs not only with [[electromagnetic waves]] such as [[light]] and [[microwave]]s, but also with other types of waves, including [[sound]] and [[water waves]]. If the waves are capable of forming a narrow beam (Fig.{{nnbsp}}2), the reflection tends to be described in terms of "[[ray (optics)|rays]]" rather than waves; in a medium whose properties are independent of direction, such as air, water or [[glass]], the "rays" are perpendicular to  associated [[wavefront]]s. The total internal reflection occurs when critical angle is exceeded.

[[File:Total internal reflection by fluorescence.jpg|thumb|'''Fig.{{nnbsp}}2''':{{big|&nbsp;}}Repeated total internal reflection of a [[blue laser|405{{nnbsp}}nm&nbsp;laser]] beam between the front and back surfaces of a glass pane. The color of the laser light itself is deep violet; but its [[wavelength]] is short enough to cause [[fluorescence]] in the glass, which re-radiates greenish light in all directions, rendering the zigzag beam visible.]]

[[Refraction]] is generally accompanied by ''partial'' reflection. When waves are refracted from a medium of lower propagation speed (higher [[refractive index]]) to a medium of higher propagation speed (lower refractive index)—e.g., from water to air—the ''[[angle of refraction]]'' (between the outgoing ray and the surface [[normal (geometry)|normal]]) is greater than the ''[[angle of incidence (optics)|angle of incidence]]'' (between the incoming ray and the normal). As the angle of incidence approaches a certain threshold, called the ''[[#Critical angle|critical angle]]'', the angle of refraction approaches 90°, at which the refracted ray becomes parallel to the boundary surface. As the angle of incidence increases beyond the critical angle, the conditions of refraction can no longer be satisfied, so there is no refracted ray, and the partial reflection becomes total. For [[visible light]], the critical angle is about 49° for incidence from water to air, and about 42° for incidence from common glass to air.

Details of the mechanism of TIR give rise to more subtle phenomena. While total reflection, by definition, involves no continuing flow of power ''across'' the interface between the two media, the external medium carries a so-called ''[[evanescent wave]]'', which travels ''along'' the interface with an amplitude that falls off exponentially with distance from the interface. The "total" reflection is indeed total if the external medium is lossless (perfectly transparent), continuous, and of infinite extent, but can be conspicuously ''less'' than total if the evanescent wave is absorbed by a lossy external medium ("[[attenuated total reflectance]]"), or diverted by the outer boundary of the external medium or by objects embedded in that medium ("frustrated"&nbsp;TIR). Unlike ''partial'' reflection between transparent media, total internal reflection is accompanied by a non-trivial [[Phase (waves)|phase shift]] (not just zero or 180°) for each component of [[polarization (waves)|polarization]] (perpendicular or parallel to the [[plane of incidence]]), and the shifts vary with the angle of incidence. The explanation of this effect by [[Augustin-Jean Fresnel]], in 1823, added to the evidence in favor of the [[wave theory of light]].

The phase shifts are used by Fresnel's invention, the [[Fresnel rhomb]], to modify polarization. The efficiency of the total internal reflection is exploited by [[optical fiber]]s (used in [[telecommunications cable]]s and in image-forming [[fiberscope]]s), and by [[Prism (optics)|reflective prism]]s, such as [[erect image|image-erecting]] [[Porro prism|Porro]]/[[roof prism]]s for [[monocular]]s and [[binoculars]].