# Colour of colloids

Why colours of colloidal solutions are different when viewed along different directions?

For example Milk appears blue when viewed by reflected light and red when viewed by transmitted light

Is it due to the irregular shape of the particles that they scatter different wavelengths along different directions?

• Could you give an example? Like milk? Viewed with the naked eye? Perhaps it is just an apparent difference because of the distance the light must traverse. Mar 9, 2019 at 17:13
• something like a redshift? Mar 9, 2019 at 17:18
• I mean, what kind of experimental setup are you considering here? Or is this an observation you made at home? Why should a colloidal solution exhibit optical anisotropy? Mar 9, 2019 at 17:21
• This is mostly due to diffraction, which normally cancels out unless the particles are really uniform. en.wikipedia.org/wiki/Mie_scattering Mar 9, 2019 at 17:25
• Also look for Tyndall effect quora.com/… and en.m.wikipedia.org/wiki/Tyndall_effect Mar 9, 2019 at 19:26

This is classic, the effect is called Mie scattering.

Very simply speaking, light is scattered on the whole surface of every (translucent) particle. For round particles, interference makes a diffraction pattern with conic symmetry, i.e. the intensity of diffracted light depends on the angle towards the incoming wave.

Obviously the pattern depends on the light wavelength, and so there is a dispersive effect. It's not very strong, because usually the particles are not all exactly of uniform size, and there is a lot of multiscattering.

Btw. it takes a keen observer to notice this without prior knowledge. Congratulations. ;-) It's easier to see if you dilute the milk (less multiple scattering). At least one good use for low-fat milk.

Warning: The explanation above is terribly simplified, to the point where it doesn't explain much. For example you'd expect the colours to change with every brand of non-/homogenised milk, because the size distribution varies. Not so much.

Also the effect is obscured by the fact that light scattering is generally much stronger for shorter wavelenghts. That also means that red light is more likely to pass through, and blue light to get absorbed or escape on the front side. It's really hard to tell apart Mie scattering and the ordinary Tyndall effect.