Tyndall effect is observed only when

the diameter of the dispersed particles is not much smaller than the wavelength of light used 

there is high difference between the refractive indices $\left(\mathrm{\mu }\right)$of the dispersion medium and dispersed phase

refractive indices $\left(\mathrm{\mu }\right)$ of both dispersed medium and dispersed phase are equal to each other

Both (a) and (b)