Total reflection

In Optical geometrical, a luminous ray crossing a separation between two mediums of optical indices different can undergo a reflection and a Réfraction. When there is no refracted ray, it is said that it undergoes a total reflection .

The Lois of Snell-Descartes make it possible to predict in which direction the considered ray is propagated (see figure opposite). They explain also the phenomenon of the total reflection.

Indeed, the law of Snell-Descartes for the refraction, n_1 \ sin \ theta_1=n_2 \ sin \ theta_2 \, (see figure), makes it possible to show that there exists a angle limits from which the refraction is impossible. That is not possible that if n_1>n_2 where N 1 and N 2 is the indexes of refraction of each medium.

On the diagram opposite, the angle θ1 is smaller than the limiting angle and the red ray at the same time considered and is refracted. For the incidental blue ray according to the angle θ2 higher than the critical angle, there is total reflection. The limiting angle is given by the law of Snell-Descartes, and one a:

\ theta_l = \ arcsin \ left (\ frac {n_2} {n_1} \ right)

The measurement of the limiting angle thus makes it possible to know the report/ratio of the index of refraction of two materials, and if one is known to measure the other. This principle is used in the Réfractomètre S.




Wave évanescente, frustrated total reflection

If the total reflection prevents the existence of a refracted wave being able to be propagated in the refringent medium, it persists there nevertheless a wave which is not propagated, in the vicinity immediate of the diopter. This Onde évanescente has a decreasing amplitude Exponentielle lies when one moves away from the diopter, and it is independent of time. The electrodynamic of the continuous mediums allows its calculation. This phenomenon is in particular used by the Microscope of fluorescence by total reflection interns.

This wave makes it possible to obtain the phenomenon of frustrated total Réflexion: by joining a second diopter in the vicinity of the first, it is possible to recover part of the wave évanescente, and all occurs as if there had not been total reflection. Indeed, part of the light is refracted.

See too

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