Tyndall effect

When light encounters small suspended particles, some of it is redirected rather than continuing in its original direction. If enough scattered light reaches an observer, the path of a beam becomes visible from the side. Smoke in a sunlit room, fog illuminated by a lamp and a searchlight beam in clouds are familiar examples. The name Tyndall effect is often used for light scattering by particles in a colloidal dispersion; the physical scattering behavior depends on particle size, the wavelength of light and the surrounding medium.

A colloid contains dispersed particles larger than ordinary molecules or ions but small enough that they generally do not settle out under gravity. Such particles can scatter visible light, making a beam detectable through the mixture. In a true solution, dissolved molecules or ions are far smaller and usually do not scatter visible light enough for a comparable beam to be seen. This distinction is useful in demonstrations and introductory chemistry, though appearance alone is not a complete analytical test: suspensions, absorption, particle concentration and other optical effects can also influence what is observed.

The Tyndall effect is related to, but should not be confused with, Rayleigh scattering. Rayleigh scattering describes scattering by particles much smaller than the wavelength of light, such as air molecules; it is especially strong at shorter wavelengths and contributes to the blue appearance of the daytime sky. Tyndall's experiments with smoke and other particles helped investigate light scattering, but the modern explanation of the blue sky emphasizes scattering by molecules of the atmosphere rather than dust or water droplets. Particle size and composition determine which scattering description is appropriate.

The effect is named for the Irish-born British physicist John Tyndall, who studied the interaction of light with gases, liquids and suspended particles in the nineteenth century. His blue-sky apparatus used a tube through which he passed light and gradually introduced smoke, producing a visible scattered beam and colored light at different viewing angles. Today the principle is used in demonstrations of colloids and in optical instruments that detect particles in aerosols or fluids. In these settings, the intensity and direction of scattered light can provide information about particles, although quantitative measurements require controlled conditions and instrument-specific analysis.