波的衍射

人们为了分析波的衍射现象，构造了许多数学模型，其中包括从波动方程推导出的菲涅耳－基尔霍夫衍射公式、夫琅禾费衍射模型以及菲涅耳衍射模型。^[24]:198-200设为圆孔半径或狭缝宽度，为入射波的波长，为观察屏距离圆孔、狭缝等衍射物体的距离，如果它们满足

http://zh.wikipedia.org/wiki/%E8%A1%8D%E5%B0%84

http://en.wikipedia.org/wiki/Diffraction

Tech Stuff - Wireless - Fresnel Zones and their Effect

Fresnel provided a means to calculate how out of phase the bumps (deflections) between the transmission source (TX) and the receptor (RX) will be. Each Fresnel zone is an ellipsoidal shape ('sausage like' to the rest of us) as shown below. Deflections (bumps) from obstacles which occur anywhere in zone 1 will, if they arrive at the receptor (RX), create signals that will be 0 to 90 ^o  out of phase, in zone 2 they will be 90 to 270 ^o  out of phase, in zone 3 they will be 270 to 450 ^o  out of phase and so on. Even numbered zones are bad'ish (they have the maximum  phase cancelling  effect) and odd numbered zones are good'ish (they may actually add to the signal power). The signal strength (and hence the magnitude of the  phase cancelling  effect) is strongest in zone 1 and decreases in each successive zone simply because wave paths get longer (and signals get weaker) the further they are from the direct line from transmitter (TX) to receptor (RX). Because the Fresnel zone is ellipsoidal, at any arbitrary point which is  d1 distance from the transmitter (TX) and  d2  distance from the receptor (RX) each Fresnel zone has a unique radius value of  r1 . Fresnel's equation allow us to calculate this unique radius ( r1 ) given the value of either  d1  or  d2 . See diagram 2.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html