Use holography to make an interferometric motion detector!
(apologies for the atrocious sound quality)
A hologram can record a near-perfect 3-D image of a real object. When this image is played back exactly on top of the object itself you see two images simultaneously, the recorded one and the real one. You can see fringes (light and dark lines) showing how the light rays from the two images interfere with each other. Each fringe cycle, from dark band to light and then back to dark, represents one additional wavelength of displacement between the two images.
This plate can also be made to vibrate by driving it with a small speaker. As the amplitude of the sound increases, the only part of the fringe pattern that remains visible is where the plate is NOT moving, on the nodal lines.
