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The Tyndall effect occurs when particles within a colloid or suspension scatter the light that passes through. The intensity of the scattering is a direct result of the size of the colloidal particles; since they are roughly the size of a single wavelength of light, the Tyndall effect is much more intense than a similar effect known as Rayleigh scattering. The most common practical application of the effect is the detection of colloids and ultramicroscopic particles. The Tyndall effect can also be used to detect light that would otherwise be invisible to the naked eye.
A common Tyndall effect demonstration involves the creation of a clear colloid, such as water-based ones, inside a transparent glass. When a beam of light passes through the glass, the beam itself is clearly and visibly delineated within the colloid. This is a result of longer wavelengths passing through the substance while shorter wavelengths of light are scattered, reflecting the shorter light back to the viewer. In some cases, the scattering can alter the perceived color of a colloid. Flour mixed with water, for example, will appear blue when prepared as a colloid; the same effect is achieved in the irises of blue-eyed individuals.
The Tyndall effect can reliably be used to detect colloids, and by extension, small particles within the colloids. Conventional microscopes have difficulty capturing images of particles smaller than 0.1 micron in size, making it a challenge to determine whether or not a particular substance is a colloid or a true solution. If a beam of light scatters when passing through a clear substance, observers can confirm the presence of particles and determine that the substance is a colloid. This principle has led to the development of ultramicroscopes, which allow scientists to observe particles that are invisible even with the aid of a traditional microscope. The same test can be used to gather an idea of the size of the particles within the colloid and its density.
The effect can also be used to detect invisible light. Since the Tyndall effect scatters light of a shorter wavelength, it is possible to render infrared light visible by passing it through a colloid. This can be achieved by blowing smoke or another gaseous colloid onto a suspected area. The particles will scatter the shorter, visible red wavelengths, allowing observers to see a beam of red light. The beam will be most visible when viewed from an angle perpendicular to the light's path.
You can see the Tyndall effect when you shine your headlights into fog. If you have your high beams on, some of the light gets scattered and sent back to you, making it hard to see. If you have your low beams on, you can see the beam in the air.
I once witnessed this effect in a scary situation. I was driving home on New Year’s Eve night, and the fog was so thick that I could not see more than two feet in front of my car.
My headlights allowed me to see the fog and the beam, but this did not help me navigate very well. I just had to pay close attention to the stripes on the road, and no matter what, I could not turn my high beams on, because that would be like looking at a yellow wall.