![]() ![]() Although a sharp shadow might have been expected, diffraction takes place once again for precisely the same reasons as before. ![]() The radiation is maximum (but not a sharp maximum if the slot is small) in front of the slot and diminishes gradually away from it.įigure 8-10 shows what happens when a plane wave meets the edge of an obstacle. It is seen that instead of being “squeezed through” as a single ray, the wave spreads out past the slot, which now acts as Huygens’ point source on a wavefront and radiates in all directions. For this to be noticeable, however, the wavefront must be small, such as that obtained with the aid of the slot in a conducting plane, as in Figure 8-9c. When a finite plane is considered, the cancellation in spurious directions is no longer complete, so that some divergence or scattering will take place. The answer is that an infinite plane wave has been considered, and mathematics shows that cancellation of the secondary wavelets will occur in all directions other than the original direction of the wavefront thus the wavefront does continue as a plane. If a plane wave is considered, as in Figure 8-9b, the question that arises immediately is Why the wavefront continues as a plane, instead of spreading out in all directions. Huygens’ principle can also be derived from Maxwell’s equations. For normal propagation, there is no need to take Huygens’ principle into account, but it must be used when Diffraction of radio waves is to be accounted for. The total field at successive points away from the source is then equal to the vector sum of these secondary wavelets. Huygens’ principle states that every point on a given (spherical) wavefront may be regarded as a source of waves from which further waves are radiated outward, in a manner as illustrated in Figure 8-9a.
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