Physics
Tyndall Effect
Quick fact
The Tyndall effect is not Rayleigh scattering (which makes the sky blue); it's scattering by larger particles—think of a laser beam passing through a glass of milk or a dusty room.
Why this is interesting
Ever wondered why a flashlight beam looks like a solid cone in fog, or why the sky changes color at sunset? The answer lies in a simple but beautiful phenomenon: the Tyndall effect.
Read the full explanation
Understanding Tyndall Effect
Imagine shining a flashlight into a glass of water mixed with a tiny drop of milk. The beam becomes visible from the side because the milk particles scatter the light in all directions. This is the Tyndall effect. It happens when light hits particles that are about the same size as the light's wavelength. When the particles are much smaller (like air molecules), we get Rayleigh scattering instead. The Tyndall effect is what makes a car's headlights visible in fog, a projector beam visible in a smoky room, or a laser pointer's path visible when there's dust in the air.
A deeper explanation
The Tyndall effect arises from the scattering of electromagnetic radiation (visible light) by particles in a colloidal system. These particles have diameters roughly 1 to 1000 nanometers—comparable to the wavelength of visible light (400–700 nm). When light encounters such a particle, it induces an oscillating electric dipole in the particle, which then re-radiates light in all directions (scattering). The amount of scattering depends on the particle size, shape, and refractive index difference with the medium. Unlike Rayleigh scattering, which is stronger for shorter wavelengths and gives the sky its blue color, Tyndall scattering is more uniform across colors and can polarize the scattered light. This effect is practically used to test whether a mixture is a true solution (no scattering) or a colloid (scattering evident). It also explains why the sky appears white on hazy days: larger water droplets or dust particles scatter all colors evenly.