Application of the Schlieren Technique to the Study of Ripples

Abstract

IN the simplest form of ripple tank demonstration, a ‘point’ source of light placed below the tank is used for casting ‘shadows’ of the ripples on a screen. The ripples have a complicated lens action on the light, so that the relation between the pattern of ripples on the water surface and the pattern of light and shade on the screen is not necessarily a simple one: caution must be exercised in interpreting the finer details of the pattern observed. In more complicated forms of the apparatus, a lens is used above the tank for projecting on to the screen a real image of the ripples; but the same difficulty arises. If the water surface is in sharp focus, the ripples are almost invisible. The lens must be raised so that it focuses the focal lines of light produced by the ripple crests. Now the distance of these focal lines above the surface of the water depends on the curvature of the ripples, and since this varies with the attenuation of the waves, it is impossible for the pattern to be in sharp focus over the screen, while obstacles introduced into the tank to produce reflexion, refraction or diffraction are always out of focus.