Tyndall Effect
Difficulty: Easy | Time: 10 minutes | Visual Impact: High
Historical Context
John Tyndall, the Irish physicist, described this scattering phenomenon in 1869 while studying why the sky is blue and why dust-laden air glows in sunbeams. He noticed that a beam of light passing through clear air leaves no visible trace, while the same beam through foggy or dusty air appears as a bright shaft.
The explanation — that particles smaller than visible wavelengths scatter light in characteristic ways — was later quantified by Lord Rayleigh (Rayleigh scattering) and extended to colloids by Richard Zsigmondy, who used the Tyndall effect in his ultramicroscope to study colloidal gold particles that were far too small to see directly. Zsigmondy received the Nobel Prize in Chemistry in 1925 for this work.
The Tyndall effect is why milk is white, why fog glows in headlights, why blue eyes are blue (they contain no blue pigment — only fine particles that scatter blue light preferentially), and why the sky is blue.
Materials
- Water — 400 mL
- Salt — 5 g (for the true solution)
- Milk — 5–10 drops (for the colloid)
- Two clear glasses or beakers — 200 mL each
- Laser pointer (any color, red is most common)
- Dark room or a dark background
Procedure
- Prepare the true solution: dissolve 5 g salt in 200 mL water — perfectly clear
- Prepare the colloid: add 5–10 drops of milk to 200 mL water — slightly cloudy or milky
- Darken the room, or position a dark background behind both glasses
- Shine the laser horizontally through the salt water — the beam is invisible inside the liquid
- Shine the laser through the milk water — the beam is clearly visible as a glowing line through the liquid
- Vary the milk concentration (more or fewer drops) to see how particle concentration affects the brightness of the beam
Extension: try other colloids — diluted coffee, cornstarch suspension, a small amount of gelatin dissolved in water.
The Science
The key difference is particle size:
| System | Particle size | Example |
|---|---|---|
| True solution | < 1 nm | Salt water, sugar water |
| Colloid | 1–1000 nm | Milk, fog, smoke |
| Suspension | > 1000 nm | Muddy water |
In a true solution, dissolved ions (Na⁺ and Cl⁻) are far smaller than the wavelength of visible light (400–700 nm). They scatter very little light — the beam passes through without revealing its path.
Milk is a colloid: tiny fat globules 100–500 nm in diameter are dispersed throughout the water. These particles are small enough not to settle out, but large enough to scatter visible light. Each globule deflects photons sideways out of the beam’s original direction, creating a glowing cone visible from the side. This is Tyndall scattering.
The scattering intensity depends on particle size and the wavelength of light (shorter wavelengths scatter more strongly — which is why scattered light often looks blue). In very dense colloids, multiple scattering produces the white appearance of milk, clouds, and fog.
The Tyndall effect is used practically in nephelometry — measuring turbidity of water supplies, blood plasma, or industrial process streams — and in dynamic light scattering instruments that determine nanoparticle sizes.