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Acousto-optic - Coggle Diagram

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        :check: Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening.
        
        :check: It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.
        
        :check: the diffraction phenomenon is described by the Huygens–Fresnel principle
        
        :check: The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image.
        
        :check: However, if there are multiple, closely spaced openings, a complex pattern of varying intensity can result.
        
        :check: These effects also occur when a light wave travels through a medium with a varying refractive index
        
        :check: The diffracting object or aperture effectively becomes a secondary source of the propagating wave.
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        Wavefront
        
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        :check: In physics the wavefront of a time-varying field is the set (locus) of all points where the wave has the same phase of the sinusoid.
        
        :check: The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal frequency (otherwise the phase is not well defined).
        
        :check: Wavefronts usually move with time. For waves propagating in an unidimensional medium, the wavefronts are usually single points; they are curves in a two dimensional medium, and surfaces in a three-dimensional one.
        
        :check: For a sinusoidal plane wave, the wavefronts are planes perpendicular to the direction of propagation, that move in that direction together with the wave.
        
        :check: For a sinusoidal spherical wave, the wavefronts are spherical surfaces that expand with it.
        
        :check: If the speed of propagation is different at different points of a wavefront, the shape and/or orientation of the wavefronts may change by refraction.
        
        :check: In particular, lenses can change the shape of optical wavefronts from planar to spherical, or vice versa.
        
        Huygens–Fresnel principle
        
        :check: It states that every point on a wavefront is itself the source of spherical wavelets, and the secondary wavelets emanating from different points mutually interfere. The sum of these spherical wavelets forms the wavefront.
        
        WAVE INTERFERENCE
        
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        :check: In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude.
        
        :check: Constructive and destructive interference result from the interaction of waves that 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.
        
        :check: The resulting images or graphs are called interferograms.
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        :check: In physics, refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium.
        
        :check: Light slows as it travels through a medium other than vacuum (such as air, glass or water). This is not because of scattering or absorption. Rather it is because, as an electromagnetic oscillation, light itself causes other electrically charged particles such as electrons, to oscillate. The oscillating electrons emit their own electromagnetic waves which interact with the original light. The resulting "combined" wave has wave packets that pass an observer at a slower rate. The light has effectively been slowed. When light returns to a vacuum and there are no electrons nearby, this slowing effect ends and its speed returns to c.
        
        :check: When light enters, exits or changes the medium it travels in, at an angle, one side or the other of the wavefront is slowed before the other. This asymmetrical slowing of the light causes it to change the angle of its travel. Once light is within the new medium with constant properties, it travels in a straight line again.
  - - - :check: Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.
- - - - :check: A piezoelectric transducer is attached to a material such as glass. An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the material.
        :check: These can be thought of as moving periodic planes of expansion and compression that change the index of refraction.
        :check: Incoming light scatters (see Brillouin scattering) off the resulting periodic index modulation and interference occurs similar to Bragg diffraction. The interaction can be thought of as a three-wave mixing process resulting in Sum-frequency generation or Difference-frequency generation between phonons and photons.
    - - :check: An acousto-optic deflector (AOD) spatially controls the optical beam.
        :check: In the operation of an acousto-optic deflector the power driving the acoustic transducer is kept on, at a constant level, while the acoustic frequency is varied to deflect the beam to different angular positions.
    - - :check: Wavelength tuning is electrically controlled through the applied RF frequencies.
    - - :check: An acousto-optical spectrometer (AOS) is based on the diffraction of light by ultrasonic waves.
        :check: A piezoelectric transducer, driven by the RF signal (from the receiver), generates an acoustic wave in a crystal (the so-called Bragg-cell).
        :check: This acoustic wave modulates the refractive index and induces a phase grating.
        :check: Depending on the crystal and the focal length of the imaging optics, the resolution of this type of spectrometer can be varied.