THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

3.8 Refraction 43of an input voltage V to the opposite faces of the piezoelectric crystal, deliversa short train of ultrasonic waves into the lens L, which may consist of a smallblock of sapphire, which incorporates a spherical hollow in the face facing liquidmedium M. The waves travel much faster in the crystal than in the adjoining liquid.As a result the central portion of each wave is retarded relative to the outerparts. All of the wavefront enter the liquid at the same time, but the refractiveeffect causes almost all of the nearly spherical waves to focus at the central pointof curvature. The strength of the reflection depends on the nature of the specimensurface at the focal point S. The operating mode of the lens and transducernow changes from the role of an emitter to that of a receptor. The lens gathersthe reflected signal and the transducer detects the signal. In this fashion this deviceresembles radar and sonar systems. As information is obtained from onlyone point at a time, the specimen must be moved in a raster pattern on the focalplane of the microscope while an image is progressively accumulated in a computermemory.If water is used as a medium, a 3-MHz signal, with a propagation velocity of1480 m/s, would have a wavelength of 500 μm = 0.5 mm, which would amountto a rather coarse resolution. Clearly, higher-frequency signals are called for, butsuch signals become strongly absorbed in water. A medium with a lower value ofc and, more importantly, less absorption than water constitutes another possibility.An attractive choice turned out to be liquid helium, used with instruments thatgenerate signals up to the 8-GHz frequency range. The wavelengths are as smallas 0.03 μm.In Figure 3.9, a geometrical ray construction illustrates the refraction of soundpassing from one medium to another. Application of Huygens’ principle leads toFigure 3.9. A sound wave passing from medium 1 to medium 2. In this case the speed ofsound c 2 in medium 2 is greater than the speed of sound c 1 in medium 1.