ULTRASONICS — LAUFER 219 



Ultrasonic waves are also used to set up space transmission gratings 

 in transparent solids, which then scatter light in the way that crystal 

 atoms scatter X-rays, resulting in diffraction patterns similar to those 

 of X-ray Laue patterns. Measurements made on these patterns permit 

 the evaluation of the longitudinal and shear velocities of sound in the 

 solid, and hence of the elastic constants of the medium. Similar 

 measurements permit the determination of the photoelastic constants 

 of the material with greater precision and far less work than was en- 

 tailed in the older interferometric methods. It should be clear from 

 the foregoing that ultrasonic research can be expected to be of use to 

 the molecular physicist, who ordinarily relies upon light or intense 

 electric and magnetic fields to produce disturbances which he can 

 measure. In ultrasonics he has a new agent, a mechanical one, with 

 which to work. 



As the intensity of the ultrasonic waves in the liquid-diffraction cell 

 is increased, more and more light is forced from the zero order into the 

 diffracted images, and at a certain sound intensity all the light is re- 

 moved from the zero order. If a slit is used to permit only the zero 

 order light to pass, the amount of light passing through the slit can be 

 controlled by the intensity of the ultrasonic waves. Ultrasonic cells, 

 which thus act as light valves, have been used as the light-modulating 

 element in sound-on-film recording systems and in the British Scoph- 

 ony system of television. Furthermore, if stationary ultrasonic waves 

 are set up in the cell by reflection, the diffraction effect is intermittent, 

 with double the frequency of the sound, the sound-wave grating being 

 created and destroyed twice each cycle. Light passing through the 

 exit slit is then modulated with this frequency and can be used to give 

 stroboscopic illumination with considerably better light output, sim- 

 pler construction, and lower electrical losses than the widely used Kerr 

 cell. A drawback, however, is the fact that the modulation frequency 

 depends upon the resonant frequency of the particular crystal used 

 and hence is not continuously variable. Still another slight change 

 in the optical system, the addition of a lens to focus the central plane of 

 the cell on a screen, permits the actual shape of the sound beam to be 

 made visible. Very clear photographs of the reflection, refraction, 

 and interference of ultrasonic waves can thus be obtained. 



A FEW APPLICATIONS 



Since the pioneer work of Wood and Loomis, each year has seen new 

 progress in ultrasonics. University and industrial laboratories inves- 

 tigated the potentialities of the new field from various directions. 

 Navy interest in sonar also stimulated ultrasonic research, and both the 

 Navy and the Army Signal Corps sponsored investigations of the prop- 

 erties and effects of high-frequency sound. Tlie results of these inves- 

 tigations indicate the unusually wide applicability of ultrasonics. 



