Incorrect statement for Tyndall effect is

The Tyndall effect is the scattering of light by colloidal particles. We need to identify the incorrect statement about it.

Recall conditions for the Tyndall effect: The Tyndall effect occurs when:

$$\bullet$$ The refractive indices of the dispersed phase and dispersion medium differ significantly.

$$\bullet$$ The diameter of the dispersed particles is not too small compared to the wavelength of light used — typically in the range of 1 nm to 1000 nm (comparable to the wavelength of visible light, 400-700 nm).

Analyze each statement: Option A: "The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude." — This is correct. A large difference in refractive index enhances scattering.

Option B: "The diameter of the dispersed particles is much smaller than the wavelength of the light used." — This is incorrect. If the particles were much smaller than the wavelength (as in true solutions), they would not scatter light effectively. For the Tyndall effect, the particle size must be comparable to or not too much smaller than the wavelength of light.

Option C: "It is used to distinguish a true solution from a colloidal solution." — This is correct. True solutions do not show the Tyndall effect, while colloidal solutions do.

Option D: "During projection of movies in the cinema hall, Tyndall effect is noticed." — This is correct. The visible beam of light from the projector passing through dusty air is an example of the Tyndall effect.

The correct answer is Option B: The diameter of the dispersed particles is much smaller than the wavelength of the light used.