Total internal reflection
Figure 1: The critical angle between total internal reflection and refraction at a glass-air boundary.
Figure 1: The critical angle between total internal reflection and refraction at a glass-air boundary.
Total internal reflection
If an energy beam strikes a media boundary at an angle α greater than Θlim, it is reflected. The angle at which such total internal reflection occurs is called the critical angle Θlim.
In the adjacent diagram, the occurrence of the critical angle and thus of a total internal reflection is shown when an electromagnetic wave front passes from an optically dense medium into an optically thinner medium. The optically denser medium is marked by the dark blue color and characterized by the (larger) refractive index n1, the thinner medium by the background color, and a smaller refractive index n2.
The critical angle is calculated according to the equation:
(1)
The total internal reflection is the cause of natural phenomena like the mirage or like apparently wet streets in the summer heat. In electromagnetic wave propagation, it is the cause of anomalies such as supra-refraction and ducting. In practice, total internal reflection is used in fiber optic cables.