Calculus allows the physicist to evaluate the infinite series and show that the wavefront of a propagating wave at any instant conforms to the envelope of spherical wavelets propagating from every point on the wavefront at the prior instant. The propagating wave front, then, can be conceptualized as shown on the left.

The first wavefront (far left) is considered as an infinite series of new field sources. Each one has its own spherical wave propagating outwards. The resulting field is represented by the interaction between all of the individual fields thus created, as seen on the right-hand side of the figure.

The problem with Huygensʼoriginal thesis was that while his ideas worked to calculate characteristics related to reflection and refraction but they donʼt account for why the wave propagates outwards. If each point generates a

spherical wave then why does the wave tend to move outwards? Why doesnʼt it just sit there and jiggle, or some such thing? Another problem is that Huygensʼ principles donʼt account for diffraction, the bending of a wave around an obstacle.

Augustine Fresnel expanded on Huygensʼoriginal theory of electromagnetic wave diffraction (no pun intended). The resulting description of wave behavior is called the Huygens-Fresnel Principle.

Some physicists downplay the significance of the Huygens-Fresnel principle in favor of the newer theories of Quantum Electrodynamics (QED) but it is nonetheless true that the principle was born of truly inspired insight and is, without doubt, a forerunner of the newer discipline of QED. It could be said that the Huygens-Fresnel principle gives the right answers for the wrong reasons.

Applying the Huygens-Fresnel Principle in the 802.11 Environment

The integration of the sum of the wavelets produced by the points on a wavefront can provide a quantification of the characteristics of the expanding electromagnetic field. With this model in mind, consider what would happen to the wavelet integration if part of the expanding field were obstructed by, perhaps, a steel girder in a building or a metal file cabinet (or a wall, or anything else that is not transparent to the 802.11 signal). In fact, when an obstruction is present, the overall affect is that the shape of the field “bends” as depicted in Figure 6.3 below.

Copyright 2003 - Joseph Bardwell