Mode conversion is the transformation of a

wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...

at an interface into other wave types (

modes Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...

Principle

Mode conversion occurs when a wave encounters an interface between materials of different impedances and the incident angle is not normal to the interface. Thus, for example, if a

longitudinal wave Longitudinal waves are waves in which the vibration of the medium is parallel ("along") to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves ...

from a fluid (e.g., water or air) strikes a solid (e.g., steel plate), it is usually refracted and reflected as a function of the angle of incidence, but if some of the energy causes particle movement in the transverse direction, a second transverse wave is generated, which can also be refracted and reflected. Snellius' law of refraction can be formulated as:

\frac =  \frac =  \frac  =  \frac

This means that the incident wave is split into two different wave types at the interface. If we consider a wave incident on an interface of two different solids (e.g. aluminum and steel), the wave type of the reflected wave also splits. Besides these simple mode conversions, an incident wave can also be converted into

surface waves In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at th ...

. For example, if one radiates a longitudinal wave at a shallower angle than that of total reflection onto a boundary surface, it will be totally reflected, but in addition a surface wave traveling along the boundary layer will be generated. The incident wave is thus converted into reflected longitudinal and surface wave. In general, mode conversions are not discrete processes, i.e. a part of the incident energy is converted into different types of waves. The amplitudes (transmission factor, reflection factor) of the converted waves depend on the angle of incidence.

Seismic waves

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, a wave conversion specifically refers to the conversion between P and S waves at discontinuities.

Body waves Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...

are reflected and refracted when they hit a boundary layer within the earth. Here, P-waves can be converted into S-waves (PS-wave) at interfaces, as well as vice versa (SP-wave). Here applies analogously for an incident P-wave:

\frac = \frac =    \frac =  \frac  =  \frac

The change in amplitudes can be described with the zoeppritz equations.

References

{{reflist Wave mechanics