The Tyndall effect is the scattering of a beam of light by colloidal particles, making the path of the light beam visible to an observer looking at right angles to the beam. When light passes through a colloid — such as milk, fog, or a starch solution — the suspended particles (size 1–1000 nm) scatter the light in all directions. This was first described by Irish physicist John Tyndall in 1869. True solutions do not show the Tyndall effect because the dissolved particles (ions or molecules, <1 nm) are too small to scatter visible light.
Named after John Tyndall, Irish physicist who described it in 1869.
Occurs when light passes through a colloid (particle size 1–1000 nm).
The scattered light is visible as a cone of light — called the Tyndall cone.
True solutions do NOT show the Tyndall effect (particle size <1 nm — too small to scatter light).
Used to distinguish colloids from true solutions.
Colloids that show it: milk, blood, starch solution, soap solution, fog, smoke, aerosols.
True solutions that do not: NaCl solution, sugar solution, CuSO₄ solution.
The scattered light is often bluish due to preferential scattering of shorter wavelengths.
The Tyndall effect occurs because colloidal particles (size range: 1–1000 nm) are large enough to scatter visible light (wavelength: 400–700 nm).
Key factors: • Particle size matters: colloidal particles (1–1000 nm) scatter light; particles in true solutions (<1 nm) are too small • Scattering: particles deflect light rays in all directions, making the beam visible from the side • The scattered light is often bluish — shorter wavelengths scatter more (Rayleigh scattering principle)
Why true solutions do NOT show the Tyndall effect: • Dissolved particles (ions or molecules) are smaller than 1 nm • Too small to interact with visible light wavelengths (400–700 nm) • Light passes through without scattering
Why suspensions scatter light but are different: • Suspension particles (>1000 nm) scatter light but the suspension is unstable — particles settle over time • Colloids are stable and do not settle
The Tyndall effect is used to distinguish a colloid from a true solution:
Colloid (shows Tyndall effect): • Particle size: 1–1000 nm • Light beam is visible as a cone (Tyndall cone) • Particles do not settle on their own • Examples: milk, blood, starch solution, soap solution, fog, aerosol, gold sol
True solution (does NOT show Tyndall effect): • Particle size: <1 nm (ions or molecules) • Light passes through without scattering — beam invisible • Completely transparent • Examples: salt water (NaCl solution), sugar solution, copper sulphate solution
Suspension (scatters light but is unstable): • Particle size: >1000 nm • Scatters light but particles settle on standing • Examples: muddy water, chalk in water
Simple test: Shine a beam of light through the sample. If the beam is clearly visible inside the liquid → colloid. If not → true solution.
Natural and everyday examples:
Fog and headlights: When car headlights shine through fog, the beam of light is clearly visible. Water droplets in fog are colloidal particles that scatter the light.
Sunlight through forest or window: When sunlight enters through gaps in curtains, windows, or through a forest canopy, the beam appears visible because of dust and moisture particles (colloidal) in the air.
Milk: Milk is a colloid (fat globules and proteins dispersed in water). When a beam of light passes through milk, it scatters — the path is visible.
Starch solution: Starch dissolved in water forms a colloid. The Tyndall effect is clearly visible, unlike a sugar solution (true solution).
Soap solution: Soap micelles in water form a colloidal solution — shows Tyndall effect.
Smoke in a room: Smoke consists of carbon particles dispersed in air — a colloid (aerosol). A beam of light through a smoky room is clearly visible.
Blue sky: The blue colour of the sky is due to scattering of sunlight by air molecules and dust particles — a related phenomenon to the Tyndall effect (Rayleigh scattering).
The Tyndall effect is the scattering of a beam of light by colloidal particles, making the path of the light visible when viewed from the side. It is named after Irish physicist John Tyndall (1869). Colloidal particles (1–1000 nm) are large enough to scatter visible light, while particles in true solutions (<1 nm) are not.
True solutions contain dissolved particles (ions or molecules) smaller than 1 nm. These particles are far too small to scatter visible light (wavelength 400–700 nm). Light passes straight through without any visible scattering. Therefore, shining a beam of light through a true solution like salt water or sugar water will not make the beam visible.
1. Fog and headlights — car headlights shining through fog produce a visible beam because water droplets scatter light. 2. Sunlight through curtains or forest — dust and moisture particles scatter the sunbeam, making it visible. 3. Milk — fat globules and protein particles in milk scatter light (milk is a colloid). All three involve colloidal particles (1–1000 nm) scattering light.
The Tyndall effect is used to: (1) Distinguish colloids from true solutions — colloids show the effect, true solutions do not. (2) Identify whether a given solution is a colloid or a true solution. (3) Estimate particle size (particles that scatter light are in the colloidal range). It is a simple and quick test in chemistry laboratories.
In Class 9 NCERT Science, the Tyndall effect is introduced while studying colloids and mixtures. It is described as the scattering of light by colloidal particles, making the path of the beam visible. The example of sunlight passing through a misty forest or fog is commonly used. It is used to distinguish colloids (like starch solution) from true solutions (like salt water).
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