Tyndall effect
/ TIN-dul /
Shine a flashlight across a glass of clear sugar water and you see nothing — the beam passes straight through, invisible. Now shine it across a glass of milky water or through misty air, and the beam suddenly lights up as a visible shaft, like a sunbeam slicing through fog. That glowing, visible path of light passing through a colloid is the Tyndall effect.
More precisely, the Tyndall effect is the scattering of a light beam by the tiny suspended particles of a colloid, which makes the beam's path visible from the side. The particles in a colloid are just the right size — comparable to the wavelength of light — to deflect light in all directions instead of letting it pass cleanly through. A true solution has no such particles, so its light passes undisturbed and the beam stays invisible.
This matters as a simple, beautiful test that tells a colloid from a true solution: shine a narrow beam, and if you can see its path, you have a colloid. It explains everyday sights too — sunbeams through dusty air or mist, and the bluish haze of headlights in fog. The caveat is that the effect needs particles in that special size range; too small (a real solution) or too big (a coarse suspension) and the telltale glowing beam fades.
Shine a laser pointer through a glass of plain water and you barely see the beam. Stir in a few drops of milk and the beam lights up as a glowing line across the glass — the milk's fat droplets scattering the light reveal it as a colloid.
A beam you can see from the side means colloidal particles are scattering it.
The Tyndall effect is closely related to the Rayleigh scattering that makes the sky blue: smaller wavelengths (blue) scatter more strongly, which is why scattered light from fine particles often looks faintly bluish.