Fog, Tyndall Effect, Headlamps ```Name: Anonymous Status: Student Age: 15 Location: N/A Country: N/A Date: August 2004 ``` Question: 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? Replies: 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 noticed 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 scattering." 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 mountains. BobErck 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 your eyes. Bob Hartwell Dear Althustler, 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... Click here to return to the Engineering Archives

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