Q.Explain what the Tyndall effect is, and explain why it is observed when a beam of light is passed through a colloidal solution but not when it is passed through a true solution.

[!TLDR] Colloidal particles are large enough to scatter visible light (Tyndall effect); true-solution particles are too small to scatter it noticeably. [!ANSWER] The Tyndall effect is the scattering of light by colloidal particles, making the light path vi …

The Tyndall effect is the scattering of a beam of light by the particles of a colloidal dispersion, which makes the beam's path visible from the side as an illuminated cone or track when viewed at an angle. It occurs because colloidal particles, in the roughly $1$–$1000\ \text{nm}$ size range, are large enough to interact with and scatter visible light effectively in many directions. A true solution's dissolved particles (individual ions or small molecules, below about $1\ \text{nm}$) are far too small compared with the wavelength of visible light (several hundred nanometres) to scatter it appreciably, so a light beam passed through a true solution remains essential …

Compare colloidal particle size against the wavelength of visible light to explain why one mixture scatt …

Do not describe the Tyndall effect as the solution 'reflecting' light like a mirror — it is scattering (redirection of light in many directions by par …