Colloids have optical properties. When a homogeneous solution is seen in the direction of light, it appears clear but it appears dark, in a perpendicular direction.
But when light passes through a colloidal solution, it is scattered in all directions. The colloidal particles absorb a portion of light and the remaining portion is scattered from the surface of the colloid. Hence the path of light is made clear. This phenomenon is known as the Tyndall effect.
