The Fresnel zone of optical gratings—the region behind the grating where secondary interference between the light coming through the different openings produces real images of the grating—can be understood by comparison with other types of wave phenomena. In the Fresnel zone of light diffracted by an optical grating, different types of amplitude modulation of the wave front are observable. Gratings of stationary ultrasonic waves, being phase gratings to the light passing through them, produce similar, though not identical Fresnel patterns. Between the planes in which the secondary interference patterns are true images of the gratings, the modulation changes from amplitude to phase modulation. This change can be demonstrated by an ultrasonic wave analog. Patterns formed by the diffraction of ultrasonic waves by an amplitude grating are given. Similar patterns in surface waves are given for both the phase grating and the amplitude grating analog.
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