Fog, Tyndall Effect, Headlamps
Date: August 2004
How can you use the Tyndall effect to describe why it is
more difficult to drive through fog with high beams than using low beams?
The Tyndall effect is caused by reflection or refraction of light by very
small particles in suspension in a transparent medium.
For example, if you shine a flashlight in a room full of smoke, you can see
the "beam." This also works for fog in air and bubbles in water. The
particles are often so small that you cannot see them with your eye, but you
can see them with a microscope.
When headlights shine through fog, some of the light is scattered back to
the driver. The glare makes it difficult to see objects on the roadway.
Some people find it more difficult to drive with the high beams on because
these shine the light straight forward and make even more light shine back
to the driver.
You will not find the term "Tyndall effect" in a physics book. Tyndall
the effect but did not know how to explain it. The explanation was done by
Gustav Mie in 1908. So the scattering of light by particles larger than
the wavelength of the light is called "Mie scattering." The scattering of
light off particles smaller than the wavelength is called "Rayleigh
Physicists have obtained an understanding of the scattering of all sorts of
things off all sorts of things. For example, the scattering of radio waves,
gamma rays, electrons, protons, etc. And the sizes range from electrons to
The Tyndall effect describes light being reflected off of smaller
particles allowing you to actually see the light "beam." For
reflection, angle of incidence equals angle of reflection. Low beams
point downward toward the road and this allows more of the reflected
light to hit the road with a smaller amount reaching your eyes. High
beams pointing higher increase the amount of reflected light reaching
The Tyndall Effect describes how light is scattered by small particles in an
otherwise transparent medium. For example, fog (small water droplets in the
air), dust in air, colloids (eg milk) in water, etc. The scattering
increases like the third power of the radius of the droplets and is stronger
for shorter wavelengths. The sky is blue because shorter wavelengths (bluer
light) is scattered more. Sunsets are red because the blue light has been
mostly scattered out leaving only the red light to reach your eyes.
To your question: I think you should use low beams when driving through
heavy fog since the headlight beams then strike the ground before going very
far. With high beams, on the other hand, the beams travel much further and
so scatter off many more droplets making the total light scattered from
water droplets much brighter. You should drive more slowly, of course,
especially since low beams don't illuminate as far ahead of the car as high
beams and, with heavy fog, you don't even see that shorter distance well.
Best, Dick Plano...
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Update: June 2012