Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : The diameter of colloidal particles in solution should not be much smaller than wavelength of light to show Tyndall effect.
Reason R : The light scatters in all directions when the size of particles is large enough.
In the light of the above statements, choose the correct answer from the options given below:

Solution

Chemistry ma’am · 4 steps

Tyndall effect is just scattering, and scattering needs particles big enough to catch the light.

1. What Tyndall effect is. When a beam of light passes through a colloid, the dispersed particles scatter it sideways. That scattered light makes the path of the beam visible from the side.
2. Check Assertion A. Scattering is noticeable only when the particle diameter is not much smaller than the wavelength of light (400 to 700 nm). Particles in a true solution are below about 1 nm, far too small, so a true solution shows no Tyndall effect. A is true.
3. Check Reason R. Colloidal particles (1 to 1000 nm) are large enough to scatter light in all directions. R is true.
4. So. R gives the very reason behind A: the size condition in A matters because only large enough particles scatter light in all directions. Both are true and R explains A, so the answer is (a).

The idea to keep. Tyndall effect needs two things: particles not much smaller than the wavelength of light, and a refractive index different from the medium. True solutions fail the size test, which is why they show no beam.

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