Incorrect statement for Tyndall effect is :

Solution

Chemistry ma’am · 3 steps

Light scatters only when the particles are big enough to catch it, so check particle size against wavelength.

1. What Tyndall effect is. When a light beam passes through a colloid, the dispersed particles scatter the light and the path of the beam becomes visible. A true solution shows no such path.
2. Two conditions. Scattering is strong only when two things hold. The particle diameter is not much smaller than the wavelength of light, and the refractive indices of the dispersed phase and the medium differ greatly.

   Statement	What it says	Verdict
   (a)	Refractive indices differ greatly	Correct, this is a required condition
   (b)	Particles much smaller than the wavelength	Incorrect, such tiny particles barely scatter, as in a true solution
   (c)	Seen during film projection in a cinema	Correct, dust and smoke make the projector beam visible
   (d)	Distinguishes true solution from colloid	Correct, only the colloid shows the beam

3. So. Statement (b) reverses the size condition. The particles must be comparable to the wavelength, not much smaller, so (b) is the incorrect statement.

The idea to keep. Tyndall effect needs particles that are not much smaller than the wavelength of light and a big refractive index difference. Particles far smaller than the wavelength, as in true solutions, do not show it.

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