The Fresnel number is a dimensionless number that occurs in optics. It is named after the physicist Augustin-Jean Fresnel. In physics, it is defined as the number of half-period zones in the wavefront amplitude, counted from the centre to the edge of the aperture, as seen from the observation point. 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, the beam is said to be near field. A Fresnel zone is defined as one of a series of confocal prolate ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system. It is used to understand and compute the strength of waves and to predict whether obstructions exist near the transmitter and or receiver.
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