PROPERTIES OF MATTER WAVES 



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Figure 3-3. Inverse Square Law. Radiation from source S diverges. Intensity (w/cm 2 ) at 

 distance, d, is four times the intensity at 2d because the same radiation is spread through 

 four times the area by the time it reaches 2d. 



However, there are two principal mechanisms of absorption of matter 

 waves by tissue: 



(a) Fnctional resistance: The momentum of the propagation, which is 

 directional (Fig. 3-1 (a)), is passed to the molecules of the tissue, which be- 

 come momentarily polarized by the pulse of pressure. The directed energy 

 thus received quickly decays into random, nondirectional molecular motion. 

 This mechanism can be called "molecular absorption." It is important at 

 medium and high frequencies. 



(b) Elastic reactance of the bulk tissue: Absorption occurs by movement of 

 the bulk material; mass is displaced, and macro-oscillations result in sym- 

 pathy with the impinging, oscillating pressure. Because the tissue is not 

 perfectly elastic (i.e., the molecules will realign themselves so that they 

 won't be polarized), the absorbed energy quickly dissipates in front of the 

 pressure pulse as molecular motion or heat. This is the only method by 

 which energy is absorbed at low frequencies — during earth tremors, train 

 rumble, or massage, for example. This mechanism can be called "elastic 

 absorption." 



Reflection, due to the inertia of the tissue (its tendency to remain at rest 

 unless forced to do otherwise — Newton's first law of motion), occurs at 



