The phenomenon where the path of the light beam is visible as a result of scattering of light by colloidal particles is called Tyndall effect. The size of the particle should be such that it should scatter light. Particles which are very small (in the case of true solutions) or very large (in the case of suspensions) do not scatter light. So, the Tyndall effect is not observed in true solutions or suspensions.
This is the general case. In certain special cases, if the particle size of suspension is small enough to scatter light, then suspensions also show Tyndall effect.
