Tyndall effect

The Tyndall effect [ tɪndl - ] describes the scattering of light by microscopic airborne particles with dimensions similar to the wavelength of light, generally in a " turbid medium ", especially in a liquid or gas are suspended. The effect is named after its discoverer, John Tyndall, who has studied the scattering of light in colloidal solutions.

Principle

By Mie scattering of light beams are scattered out the side. Thereby, the entire light beam is also visible from the side. So, take for example, in sunshine in mist or fog so-called pencil of rays is true or sees at night the light cone (Tyndall cone) of headlights in fog or clouds.

Since the effect of decreasing the wavelength increases in intensity, the stray light is often of bluish color than the primary light passing through. The effect, however, is not itself responsible for the blue color of the sky. Lord Rayleigh in 1899 as first postulated that no colloidal particles are needed for the blue of the sky, that rather, pure air, the corresponding scattering, called Rayleigh scattering caused by him. It was only in 1918 succeeded his son Robert John Strutt ( 1875-1947 ) to detect the sky-blue in an attempt.

Applications

Optical smoke detector take advantage of the Tyndall effect, using light scattered from a beam of light out of a light-sensitive sensor in the presence of smoke particles. In pure air, the light beam is invisible, and the sensor can not detect scattered light. An alarm is triggered when the sensor signal exceeds a defined threshold.

In ophthalmology, this scattering effect is used for differential diagnosis: When observing the scattering of the slit lamp light used to study inside the eye, this is a positive Tyndall effect dar. He has suspended in the aqueous humor out, for example, may pick proteins accumulate as a result of inflammatory process of the adjacent tissue structures.