Yellow Beer, White Foam

It's Friday night, and I'm stuck at home. I was supposed to go to a show in Halifax - DJ & Producer Martin Villeneuve from Ottawa was playing - but the weather was too rough for the drive, with snow and icy highways. So instead I decided to find something else to distract me, which admittedly didn't take long, since I have a huge backlog of work that I'm trying to plow through. But while I was working, I thought that it wouldn't hurt to have a glass of beer.

Have you ever really just sat and stared at a glass of beer? And if you did, did you notice anything funny? A lot of people look at things, but they don't see them. When I look at something, I like to pay attention to detail, because there are a lot of things in this world that don't make sense on the surface, if you think about them. Beer is a perfect example. If beer is yellow, then why is it that when you froth it up, the foam is white? Aha! How many people have really thought about this?

I will attempt to explain this to you, being a bit of a closet physicist (and yes, I had to think about this for a bit). You'll note that I said "closet" physicist, not "professional," so if I have any of the science wrong in this, a proper physicist can feel free to add their opinion. And also, for the sake of colour authenticity, let's assume that we're studying a bottle of Corona, rather than a dark British stout or something like that.

First, you need to know how light works. We see different colours due to many different processes. When direct light hits our eyes, the frequencies of the visible spectrum are sort of "directly" visible, as is, and if there is a combination of all frequencies, we see white (true physicists please bear with me, as this whole explanation is significantly dumbed down for the layman). In contrast, reflected light from any object is sort of the reverse concept. The object is a certain "colour" because it absorbs certain wavelengths, and the ones that it doesn't absorb are reflected away, and those are the ones that we see. So in truth, we've got a sort of "negative image" thing happening here with pigments. Anyway, that's not that important. Well, it is, but not for our beer. The point is that when we look at a beer, what we see are the frequencies of light passing through it.

Normally, a beer is illuminated by a form of white light, or an amalgam of visible frequencies. However, some of the "stuff" in beer will absorb some of those frequencies more than it absorbs other frequencies. In the particular example of beer, the constituent components tend to absorb all the frequencies except the ones that we perceive as golden yellow. So in other words, just as your kidneys filter the beer, your beer filters the light passing through it, letting the golden yellow rays through, but blocking the violets and indigos and all that other stuff.

Now you're probably wondering, "Why does this matter? Beer foam is just made of beer - if you wait long enough, it settles out. The only difference is that the foam is in the shape of bubbles, full of air." That's what I wondered for a couple minutes.

But there is the clue to help you solve the problem. The foam is full of air. In other words, there is "less beer" in the foam than there is in the liquid further down in the glass. The foam is less dense than the liquid beer. After you remember that the beer is blocking certain wavelengths, you can deduce that the foam, being less dense, is also blocking those same wavelengths, except that it is not doing as good a job because there is less absolute material that the light is being filtered through. Because of that, all the frequencies are pretty much getting through the foam, and we see white light, which is the combination of all frequencies. If you actually look very closely, slightly dense foam will have a faint yellowish tinge, and the foam that is lower and closer to the liquid beer is denser and therefore even less pure white than the "thinner" foam at the very top of the glass.

Not really that strange, when you think about it.