Tyndall Effect

Tyndall effect is a phenomenon based on the scattering of light and is named after an Irish Physicist John Tyndall. When a beam of light is passed through a colloidal solution, where the size of the constituent particles is comparable to that of the wavelength of the light beam, the beam of light is scattered in such a way that its path or trajectory becomes visible. This phenomenon of scattering of light making its path visible is termed as Tyndall effect.

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What Causes the Tyndall Effect?

Tyndall effect is seen in the colloidal solution because of the interaction of the visible spectrum of light with the constituent particles of a colloidal solution and a few fine suspensions. Therefore, the higher is the interaction between the particles and the light beam, the more is the scattering of light and the higher is the probability of seeing a Tyndall effect. 

A true solution does not show the Tyndall effect because the size of its constituent particles is smaller than 1 nm i.e. the wavelength of the visible spectrum. 

The wavelength of the visible spectrum of light falls in the range of 400 nm - 700 nm, where blue light has a wavelength of around 400 nm - 500 nm, whereas red light lies in the range of 600 nm - 700 nm. 

Now, considering the size of the constituent particles in different types of solutions:

A colloidal solution is a heterogeneous mixture in which the size of constituent particles is somewhere between 1-1000 nm, however, small enough that the constituent particles cannot be separated by the process of filtration, but centrifugation and other methods can be used because of the difference in their relative density, for example, milk. 

Since the size of the particles of a colloidal solution lies in the range of the wavelength of the visible spectrum of light, the interaction between the beam of light and the particles is good enough to scatter the beam in all directions, making its path visible. 

So, in other words, the Tyndall effect is a characteristic feature of a colloidal solution and this can easily be used to distinguish between a true solution and a colloidal solution. 

Explanation of the Tyndall Effect Through Example

Let’s take an example of a colloidal solution that shows the Tyndall effect and a true solution that does not show the Tyndall effect. Milk is an example of a colloidal solution and the class of colloids is an emulsion in which milk fat particles are dispersed in water. Unlike a true solution such as sugar dissolved in water, its constituent particles are of larger size but small enough to lie in the range of the visible spectrum of light. The optical density of milk is higher than a sugar-water solution. 

Milk fat particles cannot be separated by the physical process of filtration, however, they can be separated by the process of centrifugation, whereas sugar dissolved in water can neither be separated by the process of filtration nor by centrifugation. If asked whether milk or sugar solution (sugar dissolved in water) is a true solution or a colloidal solution, it would be really difficult to distinguish by physically looking at them. In such a situation, the Tyndall effect can be used to distinguish between the two types of solutions. 

When a beam of light is passed through the sugar solution taken in a transparent beaker or a glass bottle, the path of the light beam cannot be seen. However, when the same beam of light is torched against milk taken in a transparent glass or beaker, the path of the light beam can easily be traced along with the milk inside the beaker/glass. 

Therefore, the phenomenon of the Tyndall effect can be used to differentiate easily between the two liquids based on the nature of their constituent particles i.e. separating milk which is a colloidal solution from a sugar solution which is a true solution.

Tyndall effect is better seen when the beam of light is of a smaller wavelength such as blue light. So, red light having a higher wavelength is less scattered so shows a lesser Tyndall Effect whereas blue light shows a much better Tyndall effect.

Examples of the Tyndall Effect

Tyndall Effect has an ample number of examples and many of them can easily be seen in our day to day life. This phenomenon can easily be demonstrated at home or in schools as well.

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Some of the daily life examples of the Tyndall effect are:

• Sunlight’s path becomes visible when lots of dust particles are suspended in the air such as when the light passes through the canopy of a dense forest.

• When the weather is foggy or smoggy, the beam of headlights becomes visible.

• Sunlight enters a dark room with lots of dust particles suspended in the room.  

Some other examples of the Tyndall effect include:

• Scattering of light by water droplets in the air.

• Shinning a beam of a flashlight on a glass of milk.

• One of the most fascinating examples of the Tyndall effect is the blue-coloured iris. The translucent layer over the iris causes the scattering of the blue light making the eyes look blue. In general, this layer is opaque because of its high melanin content. But in blue eyes, this layer over the iris is translucent which helps in giving it a blue colour. 

• Mostly, the Tyndall effect is used in laboratories for determining the size of the aerosols.

On the whole, any form of colloid, whether it be sol, gel, aerosol, emulsion, foam etc. can show the Tyndall effect. This phenomenon, based on the scattering of light, lays its foundations on the concepts of general spectroscopy.

Tyndall Effect Responsible for Blue Eye Colour

The difference between the black, brown and blue coloured eyes is due to the presence of various amounts of melanin in one of the primary layers of the human eye. The amount of melanin is higher for that of the black eyes and is present in the lowest amount in blue eyes. Due to the fact that the melanin is present in the lowest amount, the iris is translucent in nature. Therefore, due to the Tyndall effect, the light gets scattered when it is incident on the translucent iris. 

Since the wavelength of the blue light is shorter than the red light, blue light scatters more than red light. As the other layer presents deep into the primary layers of the eyes absorbs the majority of unscattered lights, it causes the blue light to scatter to a greater extent. Thus, the iris gains the characteristics of blue colour.

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Several other phenomena that involve the scattering of light include Rayleigh scattering and Mie scattering. An example of Rayleigh scattering is the appearance of the blue colour of the sky due to the scattering of the light by the air particles. However, when the sky is cloudy, the scattering of light is caused by relatively large cloud droplets. This phenomenon is an example of Mie scattering.