The Tyndall effect was discovered as a result of scientists studying the interaction of light rays with various media. He found that when light rays pass through a medium containing a suspension of the smallest solid particles – for example, dusty or smoky air, colloidal solutions, cloudy glass – the scattering effect decreases as the spectral color of the beam changes from violet-blue to yellow-red part of the spectrum. If white light, for example sunlight, which contains the full color spectrum, is passed through a turbid medium, then the light in the blue part of the spectrum will be partially scattered, while the intensity of the green-yellow-red part of the light remains practically the same. Therefore, if you look at the scattered light after it passes a cloudy environment away from the light source, it will appear to us to be bluer than the original light. If we look at the light source along the scattering line, that is, through the turbid medium, the source will seem to us redder than it really is. That is why the haze from forest fires, for example, appears bluish purple to us.
The Tyndall effect occurs when scattering by suspended particles, the size of which is tens of times larger than the size of atoms. When the particles of suspension are enlarged to a size of the order of 1/20 of the wavelength of light waves (approximately from 25 nm and above), the scattering becomes polychrome, that is, the light begins to scatter evenly over the entire visible range of colors from violet to red. As a result, the Tyndall effect disappears. That is why dense fog or cumulus clouds seem to us white – they consist of a dense suspension of water dust with a particle diameter of microns to millimeters, which is significantly above the Tyndall scattering threshold.
You might think that the sky looks blue to us due to the Tyndall effect, but it is not. In the absence of cloudiness or smoke, the sky turns blue-blue due to the scattering of “daylight” on air molecules. This type of scattering is called Rayleigh scattering (in honor of Sir Rayleigh; cm. Rayleigh criterion). Rayleigh scattering scatters blue and blue light even more than Tyndall’s effect: for example, blue light with a wavelength of 400 nm is scattered in clean air nine times stronger than red light with a wavelength of 700 nm. This is why the sky looks blue to us – sunlight is scattered in the entire spectral range, but in the blue part of the spectrum it is almost an order of magnitude stronger than in the red one. The UV rays that cause sunburn are scattered even more. That is why the tan is distributed over the body quite evenly, covering even those areas of the skin that are not exposed to direct sunlight.