The Tyndall effect or Tyndall scattering is the scattering of light by small suspended particles in a colloid or fine suspension, making the light beam visible. For example, the beam of a flashlight is visible when you shine it through a glass of milk (a colloid). The effect takes its name for 19th century physicist John Tyndall, who first described and studied the phenomenon.
The Tyndall effects distinguishes colloids from true chemical solutions. The particles in a solution are very small, while those in a colloid range from 1 to 1000 nanometers in diameter. So, if you shine a flashlight beam into a glass of sugar water or salt water (solutions), the beam is not visible. However, the beam is visible in a glass of skim milk or container of gelatin (colloids).
The Tyndall effect also produces scattering in fine suspensions, such as a mixture of flour and water. However, the particles in a suspension eventually settle out, while those in a colloid remain homogeneous.
Tyndall Effect vs Rayleigh Scattering and Mie Scattering
Rayleigh scattering, the Tyndall effect, and Mie scattering all involve light scattering, but involve different particle sizes. In all three types of scattering, longer wavelengths (red) are transmitted while shorter (blue) wavelengths are reflected.
- Rayleigh scattering occurs when particles are much smaller than the wavelengths of visible light (400 to 750 nm). For example, the sky is blue due to Rayleigh scattering because the particles are tiny molecules of nitrogen and oxygen.
- The Tyndall effect occurs when particles are about the same size or smaller than the wavelengths of light. Individual particles range from 40 nm to 900 nm.
- Mie scattering occurs when particles are spherical and the same size to much larger than the wavelengths of light. For example, aerosol scattering of light in the lower atmosphere makes the area around the Sun appear white. Sunbeams produced when light passes through clouds, which contain water droplets, are also due to Mie scattering.
Examples of the Tyndall Effect
The Tyndall effect is common in everyday life. For example:
- The blue color of smoke, like from a motorcycle engine, comes from Tyndall scattering.
- The Tyndall effect causes the blue color of opals or opalescent glass, while transmitted light often appears yellow.
- Light through milk appears blue. The effect is particularly noticeable with skim milk.
- The halos around street lights come from Tyndall scattering.
- The beam from automobile lights at night, especially through fog, comes from the Tyndall effect.
- Visible rays of sunshine are sometimes due to the Tyndall effect. However, water droplets and dust motes are too large, so this example only includes fog, mist, and fine dust.
Blue Eyes and the Tyndall Effect
Blue eyes are an example of the Tyndall effect. There is no “blue” pigment in blue eyes. Rather, the iris contains much less melanin than in green, brown, or black eyes. Melanin is a pigment that absorbs light and gives the iris color. In blue eyes, light travels through a translucent layer rather than a pigmented layer. While translucent, particles in the layer scatter light. Longer wavelengths pass through the layer and are absorbed by the next layer in the iris, while shorter (blue) wavelengths are reflected back toward the front of the eye, making it appear blue.
See the Tyndall Effect Yourself
A simple demonstration of the Tyndall effect involves stirring a bit of flour or cornstarch into a glass of water and shining a flash light or laser through the light. Normally, these suspensions appear slightly off-white, yet if you shine a flashlight into the liquid it appears blue because of scattered light. Also, the flashlight beam is visible.
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