Question

The Tyndall effect is observed only when the following conditions are satisfied:

(a) The diameter of the dispersed particles is much smaller than the wavelength of the light used.

(b) The diameter of the dispersed particle is not much smaller than the wavelength of the light used.

(c) The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude.

(d) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.

• A. (a) and (c)
• B. (b) and (c)
• C. (a) and (d)
• D. (b) and (d)

Answer 1

The correct option is D

(b) and (d)

The explanation for the correct answer:-

Option (D)

1. Colloidal particles scatter light as a result of which the path of the beam becomes visible, this phenomenon of scattering light is called a Tyndall effect.
2. Faraday first observed it and later it was studied by Tyndall and hence, it is called the Tyndall effect.
3. The Tyndall effect is possible only if the following two statements are satisfied. They are:
   1. The diameter of the dispersed particles should be equal or not much smaller than the wavelength of the light used.
   2. The refractive index (i.e. the ratio of the speed of light in a vacuum to the speed of light in any medium) of the dispersed phase and the dispersion medium should differ greatly in magnitude then only the particles will be able to scatter the light and Tyndall effect will be observed.
4. So, the statements (b) and (d) are correct.

The explanation for the incorrect answer:-

In the case of option A,

1. Statement (a) says the diameter of the dispersed particles is much smaller than the wavelength of the light which does not satisfy the Tyndall effect condition as there will be no scattering of light.
2. The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude, then there will be no scattering of light and hence, therefore, no Tyndall effect is observed. So statement c also does not satisfy Tyndall
3. Hence option A is incorrect.

In the case of option B,

1. Even if statement b satisfies the Tyndall effect but statement c does not satisfy the Tyndall effect.
2. Hence option B is incorrect.

In the case of option C,

1. Even if statement d satisfies the Tyndall effect but statement a does not satisfy the Tyndall effect.
2. Hence option C is incorrect.

Hence option D is the correct option.