The Tyndall effect is the scattering of light by particles in a colloid or a particularly small solution. To be visible, the beam must be dispersed and reflected by the individual suspended particles.
The Tyndall effect can only be observed if two conditions are met:
The diameter of the dispersed particles is not significantly different from the wavelength of the light used;
The magnitudes of the dispersed phase and dispersion medium refractive indices are vastly different.