Smithsonian Report. 1951. — Laufer 



PLATE 2 



The Debye-Sears light-diffraction effect. Parallel light is beamed at right angles through 

 a cell containing a liquid in which standing ultrasonic waves have been formed by a vibrat- 

 ing crystal. The alternate regions of compression and expansion form a kind of diffraction 

 grating, but the extent to which light is passed to each of the various orders depends on 

 the amplitude of the standing waves and hence on the voltage applied to the crystal. 

 This photograph shows the diffraction spectra produced by ten megacycle waves in water. 

 The upper narrow-line spectra are obtained by using a narrow-slit aperture for the light 

 which crosses the cell. The lower broad spectra are made with a very wide slit which 

 is used in light valving. Note that at 100 volts little or no light is left in the center (or 

 zero) order. Courtesv Bell Laboratories Record. 



