The Tyndall effect is an easy way of determining whether a mixture is colloidal or not. 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.
The Tyndall effect is observed only when,

$\left(\mathrm{ii}\right)$ The diameter of the dispersed particle is not much smaller than the wavelength of the light used.

$\left(\mathrm{iv}\right)$ The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.