Dispersion (of waves)
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In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having such a property are termed dispersive media. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g. a waveguide), such as sound waves.
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Doppler Effect
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The change in frequency and wavelength of a wave for an observer moving relative to the source of the waves. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium.
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Goos-Hänchen Effect
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An optical phenomenon in which linearly polarised light undergoes a small shift, parallel to the direction of propagation, when totally internally reflected. This effect is the linear polarisation analog of the Imbert-Fedorov effect. This occurs because the reflections of a finite sized beam will interfere along a line transverse to the average propagation direction.
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Imbert-Fedorov Effect
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An optical phenomenon in which circularly or elliptically polarized light undergoes a small shift, transverse to the direction of propagation, when totally internally reflected. This effect is the circular polarisation analog of the Goos-Hänchen effect.
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Negative Refraction
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An electromagnetic phenomenon where light rays are refracted at an interface in the reverse sense to that normally expected. Negative refraction occurs at interfaces between materials at which one has an ordinary positive phase velocity (i.e. a positive refractive index), and the other has the more exotic negative phase velocity (refractive index).
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Polarisation
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A property of waves that describes the orientation of their oscillations. For transverse waves such as many electromagnetic waves, it describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel. The oscillations may be oriented in a single direction (linear polarisation), or the oscillation direction may rotate as the wave travels (circular or elliptical polarisation).
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Reflection
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The change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.
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Sagnac Effect
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(or Sagnac interference) An interferometry phenomenon elicited by rotation. A beam of light is split and the two beams made to follow a trajectory in opposite directions enclosing an area. On return to entry point the light is allowed to exit the apparatus such that an interference pattern is obtained. The position of the interference fringes is dependent on the angular velocity of the setup. The basis of ring interferometry.
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Total Internal Reflection
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An optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.
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Vibration
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Mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.
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