The conditions given below are in the context of observing Tyndall effect in colloidal solutions :
(A) The diameter of the colloidal particles is comparable to the wavelength of light used.
(B) The diameter of the colloidal particles is much smaller than the wavelength of light used.
(C) The diameter of the colloidal particles is much larger than the wavelength of light used.
(D) The refractive indices of the dispersed phase and the dispersion medium are comparable.
(E) The dispersed phase has a very different refractive index from the dispersion medium.
Choose the most appropriate conditions from the options given below :

Solution

Chemistry ma’am · 4 steps

Light scatters only when particles are wavelength-sized and bend light differently from the medium.

1. What Tyndall effect is. A beam of light becomes visible as it passes through a colloid because the particles scatter it sideways. You see the path from the side, as a bright cone.
2. Size condition. Scattering is strong only when the particle diameter is comparable to the wavelength of light. Particles much smaller, as in a true solution, scatter too little to see. Particles much larger, as in a suspension, block or reflect the light instead, and such a mixture is not a colloid anyway. So (A) is right, and (B) and (C) are out.
3. Refractive index condition. If the dispersed phase and the medium have nearly the same refractive index, light passes through the particles as if they were not there, and nothing scatters. The two refractive indices must differ a lot. So (E) is right and (D) is out.
4. So. The two conditions are (A) and (E), which is option (a).

The idea to keep. Two conditions for Tyndall effect: particle size close to the wavelength of light, and a big difference in refractive index between dispersed phase and medium. Miss either one and the beam stays invisible.

Answer from the NTA final answer key. Solution by PrepWiser.