In the interest of science a Guinness was poured in a glass until the rising bubbles had formed a creamy head.
Some of this was put in a dish and examined it through a low-powered microscope.
Unlike bath foam, which has many semi-coalesced bubbles, Guinness foam is made mainly of uniformly sized, spherical bubbles of about 0.1 to 0.2 millimeters in diameter, suspended in the good fluid itself.
Near the edge of the drop of foam it was possible to find isolated examples of bubbles, and by viewing objects held behind these it was clear that they were acting as tiny divergent lenses.
Just as a clear spherical marble, which has a higher refractive index than the surrounding air, can act as a strong magnifying glass, so spherical bubbles in beer diverge light because the air they contain has a lower refractive index than the surrounding fluid.
As a result, light entering the surface of the foam is rapidly scattered in different directions by multiple encounters with the bubbles.
Reflections from the bubbles’ surfaces also contribute to this scattering.
Some of the light finds its way back to the surface and because all wavelengths are affected in the same way we see the foam as white.
Light scattering from foam is akin to the scattering from water droplets that causes clouds to be white. This is called Mie scattering.
The glass was then drained.
On closer inspection, the head of Guinness is actually creamy colored, and a drop or two that remained in the bottom of the glass had a light brown color.
Although bulk Guinness appears black, it is not opaque.
In the foam there is not so much liquid, most of the space is taken up by air. But because light is scattered from bubble to bubble the intervening brew does absorb some of it, providing a touch of color.
Needless to say, to ensure reproducibility the experiment was repeated several times.