|MadSci Network: Chemistry|
Greetings: Reference: F. A. Jenkins, H. E. White, Fundamentals of Optics, McGraw-Hill, NY, 1957 John Tyndall (1820-1893) was a British “natural philosopher” and became superintendent of the Royal Institution in 1867. Tyndall was outstanding for his ability to popularize and clarify physical discoveries and one particular type of light scattering was named after him. The light scattering effect that Tyndall observed is a specific phenomenon, part of the more general topic of the absorption and scattering of light studied by Rayleigh. The blue sky and red and yellow sunset described by Rayleigh Scattering produces the yellow headlight which is not the effect that Tyndall observed while looking at scattered light through a polarization analyzer. The absorption and scattering of light was extensively studied during the 19th century by giants in physics including Lord Rayleigh (1842-1919) who was a professor of physics at Cambridge University and the Royal Institution of Great Britain. His works on sound and the scattering of light are the best known and he was awarded the Nobel Prize in 1904. Rayleigh scattering is related to the size of the particles in the atmosphere relative to the wavelengths contained in white light. Small particles about 400 nanometers in diameter scatter blue light with wavelengths of about the same size and pass the longer yellow, orange and red wavelengths. The referenced book describes an interesting experiment to show both the polarized and the scattered light that are the origin of the blue sky and red sunset. The experiment can also be used to observe the Tyndall effect. In the experiment, white light from the sun (or a carbon arc) is directed in a parallel beam through a very clear tank of water to which certain photographic developer chemicals are added. The chemicals form fine microscopic particles which begin to precipitate slowly in the water and the particles begin to scatter blue light (the shortest wavelength) which marks the path of the beam. As more blue scattering particles are formed and filtered out of the beam coming out of the tank, the beam comming out of the tank slowly changes color from white to yellow (the head lights) then to orange and finally red (the longest wavelengths). In the latter stages of the experiment the whole front end of the tank where the light enters is blue from the scattered light (the blue sky) and the other end is yellow orange (the setting sun). The Tyndall effect If at the beginning of the experiment described above we view the light beam from the side (at a right angle) through a polarization analyzer, the blue scattered light is first seen to be plane polarized. After more particles are formed the light becomes only partially polarized. In performing this type of experiment Tyndall was the first to observe another type of scattering when the particles become large enough to scatter white light. If the white light is viewed through a polarization analyzer at a setting where the polarized blue light is extinguished in the first experiment, the blue light will reappear with increased brilliancy in a linear polarization. This Tyndall called the “residual blue”. Rayleigh theoretically described the phenomenon and showed that with smaller particles residual blue vanishes. As you can see Tyndall’s is a fairly exotic effect relative to the yellowing of headlights by Rayleigh scattering! Best regards, your Mad Scientist Adrian Popa
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