Whether bubble bath in the bathtub, the foam on the tip of breaking waves, or the foam on a freshly tapped beer - are all bright white. The amazing thing is that the liquids that form the foam are anything but white.
Water is crystal clear; Beer - depending on the variety - yellowish or brownish. Whether we see something transparent, reflective or opaque, depends on how the material affects the incident (white) light. Liquids and other materials seem crystal clear to us when they leave the light to pass unhindered. A liquid, the light at the boundary layer to the air but also reflect in part or in whole. This depends on the angle at which the light hits the boundary layer.
This phenomenon can be observed, for example, on a windless day on a lake: If you look at a shallow angle to the smooth surface of the water, it acts like a mirror. But if one looks normal to the surface, you can see under the water surface. Foam consists of many spherical freezer paper bubbles of different sizes, freezer paper which are surrounded by a thin liquid layer. Light falls on the curved boundary layers, a portion is reflected directly. The curvature is the light which occurs at different locations, thrown back in different directions. Thus we see in each bubble a weak and distorted reflection. The other part of the light is scattered and reflected in connection many times to other bubbles or distracted again. Overall, this is a large part of the incident light is reflected. By the deflection of the many irregularly arranged bubbles, however, the reflection is done in random directions. This is known as light scattering. As a result, the foam acts white and opaque.
But why did the foam is not the color of the liquid? What color an object affects us, depends on which parts of the light transmitted therethrough and which was added by the molecules of the liquid (absorbed) are. A material absorbs the light completely, it affects us black. Some liquids (such as beer) can only certain colors unimpeded. The other colors are absorbed by the dyes contained in the beer. As a result, one sees only those portions of the light that can pass through. However, in the liquid foam layers are very thin and therefore contain only a few dyes. The light hits it on its way through the foam only a few dyes. As a result, the scattering of light plays a greater role than the absorption by the dyes. The beer foam appears freezer paper therefore mainly white.
This article I wrote in 2011 in the category "Ask the Scientist" for the ExtraTip Göttingen. The article was published on the website of the Max Planck Institute for Dynamics and Self-Organization. Items Beer ExtraTip Göttingen refraction reflection foam knowledge science newspaper Post navigation
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