Why is all soap foam white, regardless of the color of the soap?
In: Chemistry
The color “white” is what your brain uses to let you know that all the colors of the rainbow are entering your eyes at the same time from the same place and in relative amounts that approximate the balance of colors from the sun.
When you see an object that is a spectral color (such as red or blue), that is due to the fact that only a subset of the colors of light is being emitted from that object. This can be because other colors from white light have been absorbed by pigments in that object as it reflects light. The pigments in colorful soap do this; they absorb the colors that you do not see and reflect the ones you do.
Soap foam consists of a bunch of shiny liquid surfaces being held apart by trapped air. The surfaces of all these bubbles reflect almost all the colors of light in the same way that the sun shining off the surface of a lake looks white. The pigments from the soap are dissolved in the liquid, but they are not playing a big role in how light is moving around in the foam because the light is bouncing off the liquid surfaces more than it is interacting with the thin volume of liquid that is there.
Because all the surfaces have strong curvature which points the reflection in all different directions, any light that is bouncing around in soap foam gets very very mixed up. The light that escapes to get to your eyes is light that has been reflected over and over and over within the foam. Because the liquid surfaces reflect all colors roughly equally, what you see when you look at soap foam is a mixture of all the light that was shining on it. Since we use white light sources to illuminate our world, soap foam looks white.
The same explanation (tons of internal reflection) is why snow and clouds are also white, even though what they’re made of (water) is clear. But if you shine light that has a different color on snow or clouds, they’ll just reflect that color. This is why snow looks blue in the evenings and sunsets have pink and orange clouds.
TL;DR bubbles average out the color of light shone upon them because they’re a bunch of little shiny things reflecting off one another
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