Figure : Diffraction through a narrow slit.
The Huygens Principle provides one model for understanding this behavior. Imagine that at any given instant, every point on a wavefront can be considered the starting point for a spherical “wavelet”. This idea was later extended by Fresnel, and whether it adequately describes the phenomenon is still a matter of debate. But for our purposes, the Huygens model describes the effect quite well.
Figure : The Huygens Principle.
Through means of the effect of diffraction, waves will “bend” around corners or through an opening in a barrier. The wavelengths of visible light are far too small for humans to observe this effect directly. Microwaves, with a wavelength of several centimeters, will show the effects of diffraction when waves hit walls, mountain peaks, and other obstacles. It seems as if the obstruction causes the wave to change its direction and go around corners.
Figure : Diffraction over a mountain top.
Note that diffraction comes at the cost of power: the energy of the diffracted wave is significantly less than that of the wavefront that caused it. But in some very specific applications, you can take advantage of the diffraction effect to circumvent obstacles.
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