MadSci Network: Chemistry

Re: what is the 'Tyndall Effect' and how does it affect everyday life?

Date: Wed Nov 18 10:05:31 1998
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Chemistry
ID: 911101844.Ch


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 

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 

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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