Tyndall Effect
The scattering of a light beam by colloidal particles, making the beam's path visible through the medium.
Unit
Dimensionless
In depth
Particles comparable in size to the wavelength of light scatter it strongly, whereas the far smaller particles in a true solution do not. This makes the Tyndall effect a simple and reliable test to distinguish a colloid from a solution, and it explains why shorter wavelengths scatter more.
Examples in the real world
Sunbeams visible through mist or dusty air; a laser pointer's path showing clearly in milk but not in salt water.