Q.Tyndall effect is observed in

Imagine you are in a dark room with a single ray of sunlight streaming in through a tiny hole. You can see the beam of light cutting through the air — the dust motes floating in that shaft of light become visible, and the whole path of the beam glows faintly. That glowing path is the Tyndall effect in action.

The core idea is simple: light travels in straight lines, but when it hits a particle that is just the right size, it gets scattered in all directions. If the particle is too small (like individual molecules in a true solution), the scattering is negligible and the beam remains invisible from the side. If the particle is too large (like sand in water), the light is blocked or reflected, not scattered softly. But when the particle size is roughly between 1 nm and 1000 nm — the colloidal range — the scattering becomes strong enough that the beam's path lights up.

The precise statement: When a beam of light passes through a colloidal dispersion, the light is scattered by the dispersed particles. This scattered light is visible from the side, outlining the beam's path. The effect is named after the 19th-century physicist John Tyndall, who first studied it systematically.

Why does this happen? Light is an electromagnetic wave. When it encounters a particle, the electric field of the wave forces the electrons in the particle to oscillate. These oscillating charges then re-radiate light in all directions — that's the scattered light. The intensity of scattering depends strongly on the particle size relative to the wavelength of light. For particles much smaller than the wavelength (like molecules in a true solution), the scattering is extremely weak and follows Rayleigh's law (intensity $\propto 1/\lambda^4$). For colloidal particles, which are comparable in size to the wavelength, the scattering is much stronger and less wavelength-dependent — this is the Tyndall regime.