The Tyndall effect is observed only when 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 particles 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.

Options

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

Answer

To determine the conditions under which the Tyndall effect is observed, we need to analyze each of the provided statements one by one:

1. **Statement (a)**: The diameter of the dispersed particles is much smaller than the wavelength of the light used.
- This statement is **false**. The Tyndall effect occurs when the diameter of the dispersed particles is comparable to or larger than the wavelength of light. If the particles are much smaller than the wavelength, scattering of light will not be significant enough to observe the Tyndall effect.

2. **Statement (b)**: The diameter of the dispersed particles is not much smaller than the wavelength of the light used.
- This statement is **true**. For the Tyndall effect to be observed, the size of the dispersed particles should be comparable to the wavelength of the light. This allows for effective scattering of light, which is the essence of the Tyndall effect.

3. **Statement (c)**: The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude.
- This statement is **false**. For the Tyndall effect to occur, there must be a significant difference in the refractive indices of the dispersed phase and the dispersion medium. If they are similar, light will not scatter effectively.

4. **Statement (d)**: The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.
- This statement is **true**. A significant difference in refractive indices between the dispersed phase and the dispersion medium is necessary for the Tyndall effect to be observed, as it enhances the scattering of light.

### Conclusion:
The correct conditions for observing the Tyndall effect are satisfied by statements (b) and (d). Therefore, the Tyndall effect is observed when:
- The diameter of the dispersed particles is not much smaller than the wavelength of the light used (Statement b).
- The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude (Statement d).

### Final Answer:
The Tyndall effect is observed when:
- Statement (b) is true.
- Statement (d) is true.