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:

The Tyndall effect refers to the scattering of light by colloidal particles, making the path of a light beam visible when passed through a colloid. Two conditions must be met for the Tyndall effect to be observed clearly.

First, regarding particle size: the colloidal particles must have a diameter that is comparable to the wavelength of the light used (condition A). If the particles are much smaller than the wavelength (condition B), scattering is negligible (Rayleigh scattering regime is weak for very small particles relative to wavelength). If the particles are much larger than the wavelength (condition C), they simply reflect and refract light like ordinary particles rather than scatter it in the diffuse manner characteristic of the Tyndall effect. Thus condition A is required, not B or C.

Second, regarding refractive index: there must be a significant difference in the refractive index between the dispersed phase and the dispersion medium (condition E). If the refractive indices are comparable (condition D), there is very little optical contrast between the particles and the medium, so scattering is minimal and the beam path is not visible. A large refractive index difference ensures that light is strongly scattered as it encounters the particles.

Therefore, the correct conditions for observing the Tyndall effect are (A) and (E) only. The correct answer is option 1.