Colloids: The Dispersed Substance

A colloid is a substance that is microscopically dispersed throughout another substance.

An example of a colloid is milk

The dispersed-phase particles have a diameter of between approximately 1 and 1000 nanometers.Particles are normally invisible with the use of an optical microscope but they can be seen with an ultramicroscope or an electron microscope.

Types of Colloids
A common method of classifying colloids is based on the phase of the dispersed substance and what phase it is dispersed in. They types of colloids includes sol, emulsion, foam, and aerosol. 

A chart showing the different types of colloids

The Tyndall Effect
When light is shined through a true solution, the light passes cleanly through the solution, however when light is passed through a colloidal solution, the substance in the dispersed phases scatters the light in all directions, making it readily seen. This is called the Tyndall effect, also known as the Tyndall scattering. It is named after the 19th century physicist John Tyndall.

A picture showing the Tyndall effect between water and milk

Light being shined through water and milk. The light is not reflected when passing through the water because it is not a colloid. It is however reflected in all directions when it passes through the milk, which is colloidal.

Reference: 
http://en.wikipedia.org/wiki/Colloid
http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutions/Colloid