Why are soap suds always white even when the soap itself isn't?

Answer 1

Bubbles are made of water and have air inside

Water has a higher refractive index than air (it slows down light more), and because they are spherical, they act like a divergent lens, bending the light.

As the light bounces through many bubble surfaces it is bent each time- it is scattered (reflected and refracted) in all directions (including back to your eyes).

All the wavelengths (which equate to colours) act in the same way- (hardly any of the light is absorbed as the coloured water-the bubble- is so thin)

Colours are seen when white light is shone through an object (in this case coloured bath water), where wavelengths of a particular length are absorbed and others are unaffected.

Refraction occurs when light goes through an object (a rainbow is made when light refracts through water and shows how white light is made up of all different colours)

Reflection occurs when light is bounced back off a surface

As all wavelengths act the same, if you shine light on the bubbles, the same proportions of wavelengths gets back to your eyes.

As you're using white light from a bulb or the sun (which is made up of all the colours with a wide range of wavelengths)- you'll see white bubbles.

(of course if you used a red light for example- you would see red bubbles)

If the coloured liquid was very thick or had a very intense colour, the bubbles would be lightly tinged as more of the corresponding wavelengths are absorbed- but still much lighter than the bulk fluid. (e.g. bubbles on the head of a Guinness are more creamy than white because of the dark brown liquid).

I hope that this has answered your question :)

Answer 2

Because soap when sudded out into bubbles is so thin at that point that it is not concentrated enough to have color, its mostly water.

But although it does not have color, it is still not transparent! That is because of light scattering. Note that you can also see the colors of the rainbow in larger bubbles due to interference patterns which form in thin films.

See the Related Questions to the left for more light scattering.