Tyndall effect is observed when :

The diameter of the dispersed particles is not much smaller than the wavelength of the lights used. The intensity of scattered light depends on the difference between the refractive indice of the $D . P$ and $D . M$., In lyophobic colloids, this difference is appreciable and therefore the tyndal effect is quite we ll defined but in lyophilic sols the difference is very small and the tyndal effect is very weak. So, to show Tyndall effect the refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.