In Class 9 Chemistry, the Tyndall Effect is a brilliant test to distinguish between a True Solution and a Colloid. Shine a beam of light through the liquid — the answer is immediately visible.
The Tyndall Effect explains why car headlights appear as visible beams in foggy weather — the fog droplets scatter the light. On a clear night, headlight beams are invisible from the side because clean air has nothing to scatter them.
The Tyndall Effect is the scattering of a beam of light by the particles of a colloid, making the path of light visible.
When light passes through a true solution (like salt water), the tiny solute particles (ion-sized) are too small to scatter light — the beam passes through invisibly.
When light passes through a colloid (like milk or fog), the colloidal particles (1–1000 nm) are large enough to scatter light in all directions, making the beam visible as a glowing cone.
| Substance | Type |
|---|---|
| Milk | Liquid colloid (emulsion of fat in water) |
| Fog / Mist | Liquid droplets in gas (aerosol) |
| Smoke | Solid particles in gas (aerosol) |
| Blood | Colloidal suspension of cells and proteins |
| Starch solution | Sol (solid dispersed in liquid) |
| Soap solution | Colloidal micelles |
In all these, the solute particles are at the molecular/ionic level (< 1 nm) — far too small to scatter light.
It was discovered by Irish physicist **John Tyndall** in 1869. He also used it to explain why the sky is blue — air molecules scatter shorter blue wavelengths of sunlight more than longer red wavelengths.
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