Unlike solutions, colloidal suspensions exhibit light scattering. A beam of light or laser, invisible in clear air or pure water, will trace a visible path through a genuine colloidal suspension, e.g. a headlight on a car shining through fog. This is known as the Tyndall effect (after its discoverer, the 19th-century British physicist John Tyndall), and is a special instance of diffraction.
This effect is often used as a measure of the existence of a colloid. It is visible in colloids as weak as 0.1 ppm (parts per million). However, there are exceptions. For example, the effect can not be seen with milk, which is a colloid.
Tyndall scattering occurs when the dimensions of the particles that are causing the scattering are larger than the wavelength of the radiation that is scattered. It is caused by reflection of the incident radiation from the surfaces of the particles, reflection from the interior walls of the particles, and refraction and diffraction of the radiation as it passes through the particles.
Other eponyms include Tyndall beam (the light scattered by colloidal particles).