Tyndall Effect
Summary
A laser beam is invisible in salt water but glows as a bright shaft in milky water, revealing the difference between a true solution and a colloid1. You’ll be able to explain why particle size decides whether light scatters, why scattered light skews blue, and why the same effect makes the sky blue and milk white.
History
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.
Hazards & preparation
PPE: none, but respect the laser.
- Never shine the laser pointer into anyone’s eyes or at reflective surfaces — aim it horizontally through the glasses only.
- An adult should supervise children using the laser.
Disposal: the salt water and diluted milk go straight down the drain. See the Safety page.
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.
- Darken the room, or set 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 glows as a clear line through the liquid.
- Vary the milk concentration to see how particle count changes the brightness.
What you should see
In the salt water the laser passes through with no visible path — you only see the dot where it exits. In the milky water the whole beam lights up as a glowing rod you can see from the side, brighter with more milk.
| Symptom | Likely cause | Fix |
|---|---|---|
| No beam visible in either glass | Too much room light | Darken the room and use a dark background |
| Salt water also shows a beam | Water is dusty/cloudy, or salt undissolved | Filter the water; stir until the salt fully dissolves |
| Milk beam too faint or too opaque | Milk too dilute or too concentrated | Adjust drops — you want just-cloudy, not opaque |
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.
Questions to Explore
Why does particle size determine whether light is scattered? Salt ions pass light through; milk globules scatter it. What is the relationship between particle size, wavelength, and scattering, and why is there a threshold?
Hint / answer
Particles much smaller than a wavelength of light (like dissolved ions) barely disturb the wave, so almost nothing scatters. Once particles approach the wavelength of visible light (hundreds of nanometres, like fat globules) they deflect it strongly. The threshold is roughly where particle size becomes comparable to the wavelength.
Why is scattered light often blue? Shorter wavelengths scatter more strongly (intensity ∝ 1/λ⁴). How does this explain the blue sky, red sunsets, and blue eyes?
Hint / answer
Blue (short wavelength) scatters far more than red, so the sky glows with scattered blue light. At sunset the light travels through more air, scattering the blue away and leaving the reds. Blue eyes have no blue pigment — fine structures scatter blue out of otherwise brown-backed tissue.
What is the difference between scattering, absorption, and transmission? Salt water transmits, milk scatters, black ink absorbs. How does each affect what you see?
Hint / answer
Transmitted light passes straight through (clear, no beam). Scattered light is redirected sideways, so you see a glowing beam and the liquid looks cloudy. Absorbed light is soaked up and turned to heat, so the beam dims and the liquid looks dark — three different fates for the photons.
Why is milk white rather than blue? If particles scatter blue preferentially, why doesn’t milk look blue?
Hint / answer
Milk is a dense colloid, so light scatters many times before escaping. After enough bounces all colours get scattered roughly equally, and the mix of all visible colours reads as white — the same reason clouds and fog are white.
How is the Tyndall effect used to measure particle size? Dynamic light scattering watches how fast scattered light flickers as particles jostle (Brownian motion). Why does the fluctuation rate reveal particle size?
Hint / answer
Small particles get knocked around faster by Brownian motion, so the scattered light they produce flickers more rapidly; big particles move sluggishly and flicker slowly. Measuring how fast the scattered signal fluctuates therefore tells you how big the particles are.
Going further
- Test more colloids. Try diluted coffee, a cornstarch suspension, or a little dissolved gelatin — which show a beam, and which don’t?
- Colour the scattering. Use the laser on very dilute milk and look at the beam from the side versus end-on — is the scattered light bluer than the transmitted light?
- A colloid you build from scratch: precipitate magnetite nanoparticles in Ferrofluid Synthesis.