The science behind blue irises and the Tyndall effect

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According to a study conducted online, the color ‘Blue’ is allegedly the world’s most favorite color. Even if you don’t agree, according to the study every country on our planet strongly lists it as their favorite. Our history and the world around us shows the same. Blue color or blue objects have intrigued scientists and artists like Picasso and many others. Picasso had a time when he used to paint everything only in blue shades, this period is specially named as “Picasso’s Blue Period” from (1901 to 1904). Why is such that when it comes to choosing a color for painting your house, most people choose to paint it Blue? You’re probably wearing blue jeans at this very moment or have something of blue shade on you. Look around you, you’ll find many blue objects.

As much As it is easy to find it everywhere around us, blue color seems to occur very rarely in Nature. There are millions of animal species and a million different colors of animals. Can you count the number of blue animals that come to your mind? Only a few uncommon ones would’ve come across your mind like the blue butterflies, blue-colored frogs, or blue parrots. Then there is the Blue Whale or the Blue Jay, but they are not actually blue after all. Apparently not just animals blue is rare in Humans as well. Only 8% of the total population of humans have blue eyes, that’s how rare blue color comes in Nature.

The Science behind Blue Irises

As we discussed earlier, the blue color is not so popular in Nature. While imagining blue objects in nature only a few things come to mind. Like a few oddly blue feathers, some flower petals & most imaginative could be a super rare blue lobster. So it’s not wrong to say that blue is special and which makes it rare in nature. Similarly, in humans, only a few percentages of the population have blue eyes. Let’s now look at the science behind Blue Irises.

You will be surprised to know blue eyes do not contain blue pigment in them at all. Their blue color comes from a natural phenomenon called the ‘Tyndall Effect’. The same phenomenon which makes the blue sky look blue. Depending on different types of pigments, human beings exhibit numerous shades of eyes. A human eye is made up of two layers the ‘Epithelium’ which is present in the back of the eye & the ‘Stroma’ which is present in the front. Epithelium’s made up of mere two cells & it contains ‘black-brown’ pigment. The stroma on the other hand is made up of numerous ‘colorless collagen fibers’. There are few instances when stroma contains some dark pigment which is called ‘Melanin’, while sometimes it only contains an excess quantity of collagen. As fascinating as it might seem it’s only these factors that control the appearance or color of our eye.

The colored part is known as the ‘Iris’. The iris has one simple job, to limit the amount of light that passes through to the retina of the eye. To make our iris opaque, its coated with ‘Pigment Melanin’ each varying in different degrees for each individual. Melanin makes the eyes appear Dark Brown. An interesting fact is that 10,000 years ago every human being had the same dark brown eye color. As evolution took place over the coming years, mutation took place and which turned off our pigment which was present on the front of our Iris. Due to this mutation, it allowed the light in reaching the fibers of our ‘Stromal Cells’ which are present beneath. These cells allowed light to reflect as blue, green, gray, olive-colored eyes.

Let’s look at Brown eyes, people having brown eyes contain a higher content of melanin inside their stroma. This excessive melanin absorbs the majority of the light which tries to enter our eye irrespective of collagen deposit, which gives them their own dark brown color. Green eyes on the other hand do not have so much melanin inside them, they also do not have any collagen deposit. This results in the ‘Tyndall Effect’. When light tries to enter these eyes some light is absorbed inside the pigment, these particles which are present inside the stroma again ‘Scatter Light’. This is a result, an observable ‘Blue Hue’ is created, when this blue hue is combined with the brown melanin, this is the result what we call green colored eyes.

Blue eyes, the most interesting and most fascinating objects are such because their color is completely structural.

People having blue eyes are having totally colorless stroma that contains no pigment, also it does not contain any excess deposit of collagen. According to the laws of light scattering, the entire light that enters the eyes is scattered again back to the source that is the environment. This is the result of the ‘Tyndall Effect’ which creates an observable Blue Hue. More interesting is the fact that people having blue eyes entirely depends on the quantity of light which is available at the time when we have a look into them. They actually do not possess a set color.

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Tyndall Effect

The technical definition of the Tyndall effect is “The Tyndall effect is the scattering of light as a light beam passes through a Colloid”. When light passes through a specific Colloidal solution it scatters and reflects light, which makes the beam visible. Unlike the ‘Rayleigh Scattering’, in the Tyndall effect, Blue light is scattered more dominantly than the Red light. The blue color of the sky is also the result of scattering of light, but this scattering is called ‘Rayleigh Scattering’ & not due to the Tyndall Effect. This is because the particles involved in the scattering are molecules that are present in the air. The particles which cause the Tyndall effect in a colloidal are bigger than these air molecules.

When we look at the blue ocean, it appears to be blue. Which is a result of light scattering and not because our oceans contain some blue pigments. Coming back to our blue eyes, if we relate this phenomenon with our eyes. This according to scientists is known as ‘Structural Coloring’ and not similar to pigment coloring. Also, whenever you see the blue color or blue objects in nature they are actually blue due to structural coloring, they do not possess any blue color or blue pigments.

There is a butterfly named “The Blue Morpho Butterfly”. This butterfly gets its blue color from the structure of its wings. Their wings are shaped in such a rigid way that it causes sunlight to bend in such a unique way that only the blue light reaches our eye. If we were to change the shape of their wings in a very insignificant way. Even if something other than atmosphere was filled between the gaps of their wings, the blue color would cease to exist. The ‘Blue Jay’ which we were talking about gets its blue color from a similar but slightly different process. Each feather of the jay is constructed of light scattering tiny microscopic beads. These beads are spaced in a unique way such that every color of light except the blue light is canceled out.

As we can see Tyndall effect is everywhere in our daily lives. We just have to look around.

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Examples of Tyndall Effect

Before we get to the experiment we should know what a Colloidal solution is. A colloidal solution is a solution that contains a material that is evenly spread throughout the liquid. An example of a colloidal solution would be muddy water, blood, milk solution, etc. Now let’s get back at our experiment. Take two transparent glasses one filled with sugar dissolved in water solution & the other containing milk.

As we have learned until now milk is a colloidal solution while the sugar solution as being transparent is not a colloidal solution. Now take a beam of light and pass it through both of these glasses. While a beam of light is passed through the sugar solution, we are unable to see the beam of light as it easily passes through the solution without getting scattered. Similarly, now pass this beam of light through the milk solution. When done, so we can easily trace the beam of light passing through the solution. This happens due to the Tyndall Effect.

Another example would be like this. Try suspending corn starch or just plain flour in water. As the flour is slightly off-white (slightly yellow), the liquid seems to have a slight blue color as a result of the Tyndall effect. The particles present in the solution scatter blue light more dominantly than the red light. The Tyndall effect is responsible for many more miracles that happen around us, we just need to be more curious to look around. Hope this article helped you clear and fulfill your hungry for blue objects and their occurrences in nature. Don’t forget to ask yourself more such questions, our universe is very beautiful and full of mysteries that are waiting to be discovered.

Written by Lily Jameson

Lily is a writer and artist with a background in creative arts and design. She believes that both her passions complement each other really well. She often says that what she finds difficult to express with her words, she does so through her art. As a young professional in the digital world, she is well-versed in writing for all kinds of digital platforms, and has had the opportunity to have her writings published across multiple fields. She loves to write about what she knows best – art, culture, and history – and her expertise in these areas reflect in her writing.

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