Visual Observation of Surface Vibration Nodal Patterns

Abstract

SEVERAL methods for studying surface vibration modes using holography have recently been developed^1–3. These effectively contour the surface with interference fringes, which delineate points of equal amplitude of vibration. By using stroboscopic methods in conjunction with holography, the relative phase of different points can also be measured^3–5. We describe here a somewhat simpler method of vibration analysis, which does not involve the photographic process, and does not need the stability against long term drift effects that is essential to the recording of a hologram. It is, however, capable of determining only the nodal areas. It is essentially an observational method, depending on the appearance of the speckle pattern that is seen when the surface of an object is illuminated with coherent light.