A friend (seems no one else has the nerve to ask these questions here) has wondered often why soap bubbles are always white (clear) regardless of the color of the sudproducing liquid, e.g. purple bubble bath. Any ideas?
My guess is that they are too thin. We see color in liquids because they have compounds in them that absorb light at all frequencies other than the liquid’s color. So, as light passes through it we only see one color. According to Beer’s law, the amount of photo abosorbance is related to the amount of the stuff the light must travel through. A bubble is very thin, so any light that passes through it and hits your eye have little chance of being absorbed.
That explanation is true about the lack of influence of the coloring medium – the bubbles are just too darn hin.
But…
Soap bubbles AREN’T white. Or clear. Look closely and you’ll see that they are generally either blue-green or purplish, and usually you can see both colors. Only when they get very thin do they appear blue or yellow. These colors are due to “white light interference”. It’s n coincidence that you see the same colors in oil films, or in the “supernumerary” rainbow, or in a lot of other places. The classic reference is C. V. Boys’ book “Soap Bubbles”, which I believe Dover still publishes.