Tyndall effect

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The term Tyndall effect is usually applied to the effect of light scattering on particles in colloid systems, such as suspensions or emulsions. It is named after the Irish scientist John Tyndall. The Tyndall effect is used to tell the difference between the different types of mixtures, namely solution, colloid, and suspension. For example, the Tyndall effect is noticeable when car headlamps are used in fog. The light with shorter wavelengths scatters better, thus the color of scattered light has a bluish tint. This is also the reason why the sky looks blue: the light from the sun is scattered and we see the blue light because it scatters better.

This effect occurs because short wavelengths of light towards the blue end of the spectrum hit the air molecules in the earth's atmosphere and are reflected down to the earth's surface. Longer wavelengths towards the red end of the spectrum are less affected by the particles and pass on through the earth's atmosphere. This causes blue light to be reflected down to the earth's surface which makes the sky appear blue.

Blue light scatters more readily than does red light. At sunset the pathlength of the light through the atmosphere is larger than at any other time of day and is sufficiently long that the blue components of the light have undergone multiple scattering events such that the intensity at such a great viewing distance is minimal. Additionally due to the prolonged pathlength the red light shows obvious scattering effects as observed by the sky appearing different hues of red. The red light is only visible at such great distances through the atmosphere because it is less affected (scattered) by the particles of air than the blue light which is no longer visible.

Rayleigh scattering is commonly referred to, in introductory physics text books, as what is described here as the Tyndall effect.