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The near field refers to places nearby the antenna conductors, or inside any polarizable media surrounding it, where the generation and emission of electromagnetic waves can be interfered with while the field lines remain electrically attached to the antenna, hence absorption of radiation in the near field by adjacent conducting objects detectably affects the loading on the signal generator ...
In optics, the Fresnel diffraction equation for near-field diffraction is an approximation of the Kirchhoff–Fresnel diffraction that can be applied to the propagation of waves in the near field. [1] It is used to calculate the diffraction pattern created by waves passing through an aperture or around an object, when viewed from relatively ...
Self-focusing refers to the narrowing of the ultrasonic beam in the near-field. The effect occurs because, if the ultrasound wave generated by the crystal is coherent , the edges of the emitting surface generate wavelets that constructively interfere with the plane wave that is generated.
Sound field of a non focusing 4 MHz ultrasonic transducer with a near field length of N = 67 mm in water. The plot shows the sound pressure at a logarithmic db-scale. Sound pressure field of the same ultrasonic transducer (4 MHz, N = 67 mm) with the transducer surface having a spherical curvature with the curvature radius R = 30 mm
Memorial in Jena, Germany to Ernst Karl Abbe, who approximated the diffraction limit of a microscope as = , where d is the resolvable feature size, λ is the wavelength of light, n is the index of refraction of the medium being imaged in, and θ (depicted as α in the inscription) is the half-angle subtended by the optical objective lens (representing the numerical aperture).
Several examples of how Fresnel zones can be disrupted. A Fresnel zone (English: / f r eɪ ˈ n ɛ l / fray-NEL), named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver.
In optics, the Fraunhofer diffraction equation is used to model the diffraction of waves when plane waves are incident on a diffracting object, and the diffraction pattern is viewed at a sufficiently long distance (a distance satisfying Fraunhofer condition) from the object (in the far-field region), and also when it is viewed at the focal plane of an imaging lens.
Scanning near-field ultrasound holography (SNFUH) is a method for performing nondestructive nano-scale high-resolution imaging of buried and embedded structures. SNFUH is critical for analysis of materials, structures and phenomena as they continue to shrink at the micro/nano scale.