Why is foam of soap always white even if the soap is of different color?



No substance has its own color. We perceive color only when the white light, composed of different wavelengths is absorbed, reflected or scattered on their way to our eyes. For example, if a board of laminate appears red, it means that it absorbs all the colors of spectrum except red and reflects only red.


Foam of soap (also called lather of soap) is a large bunch of bubbles which are made of very thin film of soap solution plus some air. Bubbles allow some light to pass though them and scatter the rest. If no specific color is reflected, we consider this state of colorlessness as white.

More reading:
Foam (Wikipedia)


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