Speckle interference for naked-eye detection of vibrations

Abstract

An affordable optical project for students is explained in this work. The experiment only requires a handheld laser illuminating a structure with stationary vibrations induced, but the interpretation of this phenomenon demands a strong understanding of optical interference and modal behavior of a bar. The operating principle employs the speckle phenomenon, which is a type of interference between coherent light incident on a rough surface and its reflection. This interference occurs in a luminous plane outside of the vibrating object and shows a granular structure due to the brightness fluctuations. Speckle is static but if the surface illuminated oscillates, the plane of interference moves from one side to the other, causing a streak pattern. Mode shapes of the object under examination can be naked eye detected from streak patterns. What is more, this phenomenon can be photographed using a manual focus digital camera. Therefore all the components of the procedure can be available (or acquired) by almost any physics laboratory, at any level, which would be unthinkable for optical experiments just a few years ago.