Question

Tyndall effect is applicable when:
The diameter of the dispersed particle is not much smaller than the wavelength of light used.
B. the diameter of the dispersed particle is much smaller than the wavelength of light used.
C. The refractive indices of the dispersed phase and the dispersion medium are the same.
D. the refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.

Answer 1

Hint: Tyndall effect is an optical property of colloids. When an intense converging beam of light is passed through a colloidal solution kept in dark, the path of the beam gets illuminated with a bluish light. This phenomenon is called tyndall effect and the illuminated path is called tyndall cone. It occurs due to the scattering of light by colloidal particles.

Complete step by step answer:
The colloidal system consists of two phases. They are dispersed phase and dispersion medium. The medium in which the colloidal particles are distributed is called the dispersion medium. Dispersed phase is the phase in which finely divided particles are present.
-Characteristics of dispersed phase are particle size, particle shape, surface area and surface charge
Particle size influences the color of dispersion. Wavelength of light absorbed is inversely proportional to the radius of the particle. If the diameter of the particles are small, it should scatter at a very high wavelength. Therefore the particle size should not be much smaller.
And in the case of refractive index, when refractive indices of both are same, light will not get refracted since both mediums are same. So the refractive indices have to differ greatly so that light will get refracted.
Hence option A and D are correct.

Additional information:
Colloids are dispersions where dispersed particles are distributed uniformly in the dispersion medium. The dispersed phase particles have a diameter between approximately $1{\text{nm}} - 100{\text{nm}}$. Such particles are normally invisible.

Note:
Optical properties like tyndall effect are used to measure size, shape, structure, molecular weight of colloids, light scattering and turbidity. When light is passed through the dispersed particles, it polarizes the atoms or molecules and forms dipoles. This emits light in all directions. Thus light scattering occurs.