Acousto-optic Effect
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A specific case of photoelasticity, where there is a change of a material's permittivity due to a mechanical strain resulting from an acoustic wave which has been excited within a transparent medium giving rise to variation of the refractive index. This creates a diffraction grating moving with the velocity given by the spread on the sound wave in the medium which defracts light forming a prominent diffraction pattern.
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Aerosol
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A suspension of fine solid particles or liquid droplets in a gas.
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Auxetic Materials
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Materials which, when stretched, become thicker perpendicularly to the applied force, i.e. they have a negative Poisson's ratio. Such materials have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material and sponge mops.
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For example, a pressure field.
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Birefringence
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Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarisation of the light. This effect can occur only if the structure of the material is anisotropic (directionally dependent).
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Bragg Diffraction
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Diffraction from a three dimensional periodic structure such as atoms in a crystal. It is similar to what occurs when waves are scattered from a diffraction grating. Bragg diffraction is a consequence of interference between waves reflecting from different crystal planes.
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Bubble
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A globule of one substance in another, usually gas in a liquid. Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.
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By acting as a lens.
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Conduction (electrical)
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The movement of electrically charged particles through a transmission medium (electrical conductor). The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is, by diffusion .
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Electric current can be directed through conductors of any shape.
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Coriolis Force
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An apparent deflection, or force, experienced by moving objects when viewed from a rotating frame of reference. The Coriolis force is an example of a fictitious force (or pseudo force), because it does not appear when the motion is expressed in an inertial frame of reference. For example, freely moving objects on the surface of the Earth experience a Coriolis force, and appear to veer to the right in the northern hemisphere, and to the left in the southern.
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Creeping Wave
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When a wave front passes an obstruction, it spreads out into the shadowed space. A creeping wave, in electromagnetism or acoustics is the wave that is diffracted around the shadowed surface of a smooth body such as a sphere.
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Diffraction
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Various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Very similar effects are observed when there is an alteration in the properties of the medium in which the wave is travelling, for example a variation in refractive index for light waves or in acoustic impedance for sound waves and these can also be referred to as diffraction effects.
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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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Electric Field
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The space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field (that can also be equated to electric flux density). This electric field exerts a force on other electrically charged objects.
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Electro-Optic Effects
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A change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light. The term encompasses a number of distinct phenomena, which can be subdivided into a) change of the absorption (electroabsorption, Franz-Keldysh effect, Quantum-confined Stark effect, electro-chromatic effect) and b) change of the refractive index(Pockels effect, Kerr Effect, electro-gyration)
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Electrostatic Induction
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A redistribution of electrical charge in an object, caused by the influence of nearby charges. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Electrostatic induction should not be confused with electromagnetic induction; both are often referred to as 'induction'.
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Electrostatics
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The phenomena arising from stationary or slowly moving electric charges
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Faraday Effect
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An interaction between light and a magnetic field in a dielectric material. The rotation of the plane of polarisation is proportional to the intensity of the component of the magnetic field in the direction of the beam of light.
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Ferromagnetism
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The mechanism by which certain materials (such as iron) form permanent magnets and/or exhibit strong interactions with magnets. Responsible for commonly observed magnetism phenomena, e.g. 'fridge magnets. A material is 'ferromagnetic' only if all its magnetic ions add a positive contribution to the net magnetisation. If some of them subtract from the net magnetisation (i.e. are partially anti-aligned), then the material is 'ferrimagnetic'.
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Focusing
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The gathering of wavefronts of a wave (e.g. radiation) into a spherical or cylindical shape. Focusing of light is used in optics, however focusing can be applied to any radiation or wave.
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Fresnel Diffraction
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(or near-field diffraction) A process of diffraction which occurs when a wave passes through an aperture and diffracts in the near field, causing any diffraction pattern observed to differ in size and shape, depending on the distance between the aperture and the projection. It occurs due to the short distance in which the diffracted waves propagate. When the distance is increased, outgoing diffracted waves become planar and Fraunhofer diffraction occurs.
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Gravitation
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A natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass.
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Gravitational Lensing
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A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is 'bent' around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general theory of relativity.
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Holes
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A hole is a confined lack of structure in some part of an object.
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By diffraction
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Interference
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The addition (superposition) of two or more waves that result in a new wave pattern. As most commonly used, the term interference usually refers to the interaction of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.
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For example, using a diffraction grating.
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Kármán Vortex Street
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A repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies. It is responsible for such phenomena as the 'singing' of suspended telephone or power lines, the vibration of a car antenna at certain speeds.
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Kerr Effect
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(or the quadratic electro-optic effect) A change in the refractive index of a material in response to an electric field. Distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric field instead of to the magnitude of the field. All materials show a Kerr effect, but certain liquids display the effect more strongly than other materials.
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Liquid Crystals
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Substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal.
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Magnetic Field
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A vector field which surrounds magnets and electric currents, and is detected by the force it exerts on moving electric charges and on magnetic materials. When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field. Magnetic fields also have their own energy with an energy density proportional to the square of the field intensity.
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Magnetic Reluctance
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Analogous to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.
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Magnetism
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One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
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Magnetohydrodynamic Effect
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A magnetic field can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself.
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Mirage (photothermal deflection)
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A naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky.
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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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Pockels Effect
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Produces birefringence in an optical medium induced by a constant or varying electric field. Used to make Pockels cells, which are voltage-controlled wave plates.
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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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Refraction
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The change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth.
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Scattering
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A general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass (includes deviation of reflected radiation from the angle predicted by the law of reflection).
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Temperature Gradient
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The variation in temperature over distance.
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For example by creating a 'lens' based on variations in density due to termperature.
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Thermal Radiation
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Electromagnetic radiation emitted from the surface of an object due to the object's temperature.
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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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Triboelectric Effect
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A type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.
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Turbulence
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A fluid regime characterised by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time (flow that is not turbulent is called laminar flow).
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Though turbulence in the medium carrying the field.
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Voigt Effect
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A magneto-optical phenomenon whereby the polarisation of light may be rotated when passed through a vapour cell immersed in a magnetic field directed perpendicular to the beam direction.
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