Why do soap bubbles look so white

A. Soap foam is made up of tiny air bubbles encircled by thin films of soap and water. As light enters the foam it's reflected back from these film surfaces. Normally, when light changes from moving through air to moving through another substance, like water, it loses some of its speed and not all of the light makes it through the surface or is reflected back.

However, because soap foam contains so many film surfaces, most of the light is reflected back, and because the surfaces are curved, light is bounced back evenly in all directions, with no preferred wavelength (colour) making it appear white.

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Q. Why, when washing my hands with, for example pink soap, isn't the lather pink

A. Coloured soap just doesn't contain enough dye to be able to produce anything other than white lather or foam.

Q. But it contains enough dye to make the soap appear pink to my eyes

A. Yes, soap contains relatively little dye but because light travels deep into a solid before it is reflected back it's enough dye to absorb the right amount of light to influence the colour that it reflected back, and we see pink soap.

Q. Is it possible to make coloured soap that produces a coloured lather

A. Yes, but it would need so much dye that you'd probably end with stained hands as well as a coloured lather, making washing your hands with soap pointless

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by Lisa Cardy