Answer:
Scattering of light through a solution when light passes through it is called Tyndall effect.
In a colloidal solution, the particle size is between 1 nm to 100 nm, when light rays fall on colloidal solution, these particles sizes allow scattering of light ray.
But in true solution the size of particles is smaller than 1nm and unable to scatter the light and do not show tyndall effect.
Hence, colloidal solution shows tyndall effect but true solution do not.