Search results
Results from the WOW.Com Content Network
Adopted wavelength for propagation is 1 μm. The Fresnel number is a useful concept in physical optics. The Fresnel number establishes a coarse criterion to define the near and far field approximations. Essentially, if Fresnel number is small – less than roughly 1 – the beam is said to be in the far field. If Fresnel number is larger than 1 ...
A Fresnel lens (/ ˈfreɪnɛl, - nəl / FRAY-nel, -nəl; / ˈfrɛnɛl, - əl / FREN-el, -əl; or / freɪˈnɛl / fray-NEL[ 1 ]) is a type of composite compact lens which reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections.
A characteristic length is usually the volume of a system divided by its surface: [2] For example, it is used to calculate flow through circular and non-circular tubes in order to examine flow conditions (i.e., the Reynolds number). In those cases, the characteristic length is the diameter of the pipe or, in case of non-circular tubes, its hydraulic diameter : Where is the cross-sectional area ...
In fluid dynamics, Sauter mean diameter (SMD) is an average measure of particle size. It was originally developed by German scientist Josef Sauter in the late 1920s. [1][2] It is defined as the diameter of a sphere that has the same volume/surface area ratio as a particle of interest. Several methods have been devised to obtain a good estimate ...
The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media. They were deduced by French engineer and physicist Augustin-Jean Fresnel (/ freɪˈnɛl /) who was the first to understand that light is a transverse wave, when no one realized that the waves ...
Using , , and in the above formula gives where ≈ 2.997 × 108 m/s is the speed of light in the air. Substitution of the numeric value for followed by a unit conversion results in an easy way to calculate the radius of the first Fresnel zone , knowing the distance between the two antennas and the frequency of the transmitted signal :
The spot is sometimes called Fresnel's spot because it is a direct consequence of his work, and Arago's spot because Arago devised the experiment that confirmed its existence." Katz, Debora M., Physics for Scientists and Engineers: Foundations and Connections, Advance Edition, Volume 2, Cengage Learning, 2015. ISBN 1305537203 ^ "Poisson's Spot".
Near- and far-field regions for an antenna larger (diameter or length D) than the wavelength of the radiation it emits, so that D⁄λ ≫ 1. Examples are radar dishes, satellite dish antennas, radio telescopes, and other highly directional antennas.