Tyndall effect is not observed in:

Options

A. Smoke
B. Emulsions
C. Sugar solution
D. Gold sol

Explanation

To determine where the Tyndall effect is not observed among the options given (smoke, emulsion, sugar, and gold salt), we need to understand what the Tyndall effect is and the size of particles involved in each case.

### Step-by-Step Solution:

1. **Understanding the Tyndall Effect**:
- The Tyndall effect is the scattering of light by small particles in a colloid or in very fine suspensions. It is observed when a beam of light passes through a medium containing small suspended particles, making the path of the light visible.

2. **Analyzing the Options**:
- **Smoke**: Smoke consists of tiny particles (typically >1000 nm) suspended in air. The Tyndall effect is observed in smoke because the particle size is suitable for scattering light.
- **Emulsion**: An emulsion is a mixture of two immiscible liquids where one is dispersed in the other. The particle size in emulsions is generally less than 1 nm, which is too small for the Tyndall effect to be observed.
- **Sugar Solution**: A sugar solution contains sugar molecules that can range from 1 nm to 1000 nm. The Tyndall effect can be observed in sugar solutions if the concentration is high enough to create a colloidal suspension.
- **Gold Salt**: Gold salts can form colloidal solutions where the particle size is also in the range of 1 nm to 1000 nm. The Tyndall effect can be observed in these solutions as well.

3. **Conclusion**:
- Among the options, the Tyndall effect is **not observed in emulsions** due to the very small particle size (less than 1 nm) which does not scatter light effectively.

### Final Answer:
The Tyndall effect is not observed in **emulsion**.

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