The correct Answer is A colloidal solution
Explanation
The visibility of a light beam's path is determined by the scattering of light by particles present in the medium. This phenomenon is known as the Tyndall effect.
Statement-wise Analysis
- A true solution - Incorrect
True solutions contain solute particles that are too small (typically less than 1 nm in diameter) to scatter light effectively. Therefore, the path of a light beam passing through a true solution is generally not visible.
- A vacuum tube - Incorrect
A vacuum tube contains no matter or particles to scatter light. Light travels through a vacuum without scattering, making its path invisible.
- A colloidal solution - Correct
Colloidal solutions (or colloids) contain particles that are larger than those in true solutions (typically 1 nm to 1000 nm in diameter) but smaller than those in suspensions. These colloidal particles are large enough to scatter light, making the path of a light beam visible. This phenomenon is called the Tyndall effect.
- Pure glass block - Incorrect
A pure glass block is a transparent solid medium. Light passes through it without significant scattering, similar to a true solution, making the path of the light beam invisible.
Key Takeaway
The visibility of a light beam's path is due to the scattering of light by particles, a phenomenon prominently observed in colloidal solutions (Tyndall effect).