Tyndall effect is observed when:

Solution

Chemistry ma’am · 3 steps

Light scatters well only when the particle size is close to the light's wavelength. That is colloid territory.

1. What Tyndall means. When a beam of light passes through a colloid, the dispersed particles scatter it sideways, so the path of the beam becomes visible. This bright path is called the Tyndall cone.
2. Size decides scattering. A particle scatters light strongly when its size is close to the wavelength of that light. Visible light is about 400 to 700 nm, and colloidal particles are about 1 to 1000 nm, so the sizes overlap.

   Mixture	Particle size	Compared with wavelength	Tyndall effect
   True solution	below 1 nm	much smaller	no, light passes through
   Colloid	1 to 1000 nm	similar	yes, beam path glows
   Suspension	above 1000 nm	much larger	not the colloidal effect, particles block light and settle

3. So. The Tyndall effect is seen when the diameter of the dispersed particles is similar to the wavelength of the light used, which is option (a).

The idea to keep. Tyndall effect needs two things: particle size not much smaller than the wavelength of light, and a large difference in refractive index between the dispersed phase and the medium.

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