Why is the water foam white


Why is foam white?


Whether bath foam in the bathtub, the foam on the tip of breaking waves or the head of foam on a freshly tapped beer - all are radiant white. The amazing thing is that the liquids that make up the foam are anything but white.

Water is completely clear; Beer - depending on the type - yellowish or brownish. Whether we see something transparent, reflective or opaque depends on how the material influences the (white) light that hits it. Liquids and other materials appear crystal clear to us when they allow light to pass through unhindered. A liquid can also partially or completely reflect the light at the boundary layer to the air. This depends on the angle at which the light hits the boundary layer. This phenomenon can be observed, for example, on a calm day at a lake: If you look at the smooth surface of the water at a flat angle, it acts like a mirror. But if you look vertically at the surface, you can see below the surface of the water. Foam consists of many spherical air bubbles of different sizes that are surrounded by a thin layer of liquid. If light falls on these curved boundary layers, a part is reflected directly. Due to the curvature, the light, which occurs in different places, is reflected back in different directions. As a result, we see a faint and distorted reflection in every single soap bubble. The other part of the light is deflected and can then be reflected many times on other soap bubbles or deflected again. Overall, a large part of the incident light is reflected as a result. However, due to the deflection of the many irregularly arranged soap bubbles, the reflection happens in random directions. One speaks of light scattering. This makes the foam appear white and opaque. But why is the foam not the color of the liquid? The color in which an object affects us depends on which parts of the light are allowed to pass through and which are taken up (absorbed) by the molecules of the liquid. If a material completely absorbs the light, it appears black to us. Some liquids (such as beer) only allow certain colors to pass through unhindered. The remaining colors are absorbed by the dyes contained in the beer. As a result, you only see those parts of the light that can pass unhindered. In the foam, however, the liquid layers are very thin and therefore contain only a few dyes. The light hits only a few dyes on its way through the foam. As a result, the scattering of the light plays a greater role than the absorption by the dyes. The beer foam therefore appears mainly white.