I was reading in first floor of my house where sunlight was directly falling on a plastic water bottles' surface and the image of the glowing surface was projected on wall opposite to that. The wonder was that this image would oscillate with even the faintest wave reaching to it. Like a speeding car at about 40-50 km speed par hr moved through 50 ft distance from the bottle created significant oscillation of the image. I think this type of machinery system can be employed in detecting very weak mechanical waves. And also it seems to be a very fine scale.