What is the Tyndall Effect?
The Tyndall effect is the phenomenon in which the particles in a colloid scatter the beams of light that are directed at them. This effect is exhibited by all colloidal solutions and some very fine suspensions. Therefore, it can be used to verify if a given solution is a colloid. The intensity of scattered light depends on the density of the colloidal particles as well as the frequency of the incident light.
When a beam of light passes through a colloid, the colloidal particles present in the solution do not allow the beam to completely pass through. The light collides with the colloidal particles and is scattered (it deviates from its normal trajectory, which is a straight line). This scattering makes the path of the light beam visible, as illustrated below.
Generally, blue light is scattered to a greater extent when compared to red light. This is because the wavelength of blue light is smaller than that of red light. This is the reason why the smoke released by motorcycles sometimes appears blue.
The Tyndall effect was first discovered by (and is named after) the Irish physicist John Tyndall. The diameters of the particles that cause the Tyndall effect can range from 40 to 900 nanometers (1 nanometer = 10-9 meter). In comparison, the wavelength of the visible light spectrum ranges from 400 to 750 nanometers.
Examples of the Tyndall Effect
- Milk is a colloid that contains globules of fat and protein. When a beam of light is directed at a glass of milk, the light is scattered. This is a great example of the Tyndall effect.
- When a torch is switched on in a foggy environment, the path of the light becomes visible. In this scenario, the water droplets in the fog are responsible for the light scattering.
- Opalescent glass has a bluish appearance when viewed from the side. However, orange-colored light emerges when light is shined through the glass.
How is the Tyndall Effect Responsible for Blue Eye Colour?
The primary difference between blue, brown, and black coloured irises is the amount of melanin in one of its layers. The layer in a blue iris has relatively lower amounts of melanin in it when compared to a black iris, making it translucent. When light is incident on this translucent layer, it is scattered due to the Tyndall effect.
Since blue light has a shorter wavelength when compared to red light, it is scattered to a greater extent. Another layer deeper in the iris absorbs the unscattered light. Since the majority of the scattered light is blue, these irises gain their characteristic blue colour.
Several phenomena involve the scattering of light. Rayleigh scattering and Mie scattering are examples of such phenomena. Clear sky is blue due to the scattering of light by air particles, which is an example of Rayleigh scattering. However, when the sky is cloudy, the relatively large cloud droplets are responsible for the scattering of light, which is an example of Mie scattering.
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