Why is it that no matter what colour the toothpaste is when it comes out of the tube, once you’ve brushed it onto your teeth, the resulting foam is always white?
Ever really examined a soap bubble? You can see all sorts of colors on the surface, which are caused by interferece between light reflected by the outer surface and light reflected by the inner surface. The different colors are caused by certain wavelengths cancelling out, and the wavelengths that cancel are dependent upon the thickness of the film. Now, imagine very, very tiny versions of these bubbles, packed closely together, with varying film thicknesses. What you have there is various colors all being reflected more or less equally–which is the very definition of white light.
Specular reflection off the surface of a liquid is generally white. Take a look at the ceiling light reflected in your mug of coffee - it’s white, not brown. You can only see color if light is scattered by particles in the liquid, or if light penetrates deep through the liquid. Foam is mostly air so light doesn’t penetrate through much toothpaste before getting reflected by the many surfaces (air/liquid boundaries) within the foam.
two truly informative posts. thank you.
Thanks for the info. It seems that this applies to any bubbly or foaming substance. (Foam is just a lot of bubbles?)
My wife uses a special high fluoride toothpaste (Fluoridex) from her dentist that is a dark blue-green gel, and it produces a light green foam when brushing.
yes, and a bubble is a terribly thin film of water, modified to be able to stretch (often by a detergent/surfactant) around a pocket of air. In the absence of the added chemical, the water would just stick together, as you observe it doing in drops on waxed paper, or a recently waxed car, etc. and you wouldn’t get bubbles (foam).
Yeah, very intensely colored toothpastes seem to produce very lightly colored foam rather than pure white. The explanation about how foam is largely opaque that scr4 gave above still makes perfect sense - it’s just that it’s not completely opaque, so very dark toothpaste can still color the light enough to be noticeable.
Crest Cinnamon Rush makes red foam.
I used red toothpaste once. Then I freaked out because for a second I thought I had the world’s worst case of gingivitis. Seriously, red is a very, very bad color for toothpaste.
Isn’t that because there’s enough coloring agent in water of the films that make the bubbles to scatter some incoming light to make the reflected light have a colored cast?
:smack: “in the water of the films that make the bubbles…”