Total internal reflection#Critical angle

The critical ( incidence ) angle is a term from physics and is used in connection with the refraction of waves. Commonly the term is especially in the optical and in the refraction.

Description

The refraction of waves, that is, the change in direction of the gradient in the transition of a wave from one medium into an adjacent medium, based on a change of its propagation speed. This is determined by the crossed medium. Consider for example an electromagnetic wave ( such as visible light ), which propagates in a medium 1 and is incident at a known angle of incidence to a boundary surface to a medium 2. Then this will be reflected or transmitted in accordance with the Fresnel equations part, that is, the light passes into the medium 2. In this transition, the light beam experiences (which may be regarded as interference of light waves ) according to the Snell's law of refraction, a change in direction, it is "broken" (green beam path in the picture).

This means for a transition from a medium 1 into an optically denser medium 2 (,) that the beam is broken executed to Lot, for example, the transition of air in water., Are here for the propagation velocities and, for the refractive indices of the two media.

In the opposite case ( the light beam comes out of the water ), however, it is broken away from the perpendicular. If you increase now the angle of incidence, the refracted ray passes from a certain value parallel to the interface (yellow beam path ). This angle is called the critical angle of total reflection, or a critical angle. The angle of total reflection can be calculated using Snell's law of refraction:

The dependence of the beam path of the propagation speed is not liable for seismic waves. In this case, the mechanical wave propagation, the " wave beam " the vertical trajectory of the wave front dar. now Modifies the propagation velocity, thus the position of the wave front and thus the beam path is changed. Since a refractive index of mechanical waves do not exist, the Snell's law of refraction in the propagation velocities of the seismic (,) is passed through the media, defined as:

The critical angle is therefore to be calculated from:

In contrast to the electromagnetic waves, whose speed of propagation - such as described in the example of a light beam, - to decrease the optically denser medium, it increases in seismic waves when the wave front occurs in a more compact device. A critical refraction therefore occurs when seismic waves on the transition from a looser in a solid rocks.

The effect of seismic critical refraction is utilized in the method of refraction specifically for the study of layered structures of the Earth: In general, the seismic velocities increase with increasing depth so that when hitting a seismic waves on a layer boundary below the critical angle of a refracted wave arises. This so-called head wave propagates along the layer boundary with the speed of the lower layer. In this case, it continuously emits seismic wave energy, which in turn passes under the critical angle back to the surface where they are recorded and can be used to determine the velocity of propagation.