In short
The Tyndall effect is the scattering of light by tiny particles spread through a gas or a liquid, which makes the path of a light beam visible.
The particles must be small but larger than molecules, as in smoke, mist, dust or a colloid such as milk in water. A true solution, such as salt in water, shows no beam because its particles are too small. Blue light is scattered more than red, which is why the sky looks blue and the setting Sun looks red.
A torch shone through a glass of clear salt water shows no beam. Shone through a glass of water with a few drops of milk, the beam shows clearly as a bright path.
Also searched as: scattering of light, tyndall scattering, scatter light.
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