We’ve all seen brown eyes, blue eyes and even green eyes but what about all the other
colors?
To explain this, let’s look at the structure of the eye.
If we look at a cross section of the eye, we can see the iris here.
Light travels through the hole made by the iris, called the pupil.
Then the lens focuses the light onto the retina, at the back of the eye, which converts the
light to an electrical signal and sends it to your brain.
Now if we zoom in and look at a cross section of the iris, we can see that it’s made up
of three layers.
At the front, there’s the stromal layer, which is predominantly made of strands of
collagen but also has melanin, blood vessels and nerves.
Melanin is the pigment molecule produced by melanocytes that gives color to our hair,
skin and irises.
This is the most important layer for eye color which we’ll talk more about later.
Then we have the muscle layer which is made up of two muscles: the sphincter pupillae
and the dilator pupillae.
The sphincter pupillae constricts the eye when there’s a lot of light and the dilator
pupillae dilates the eye when there’s not enough.
Both are made of smooth muscle meaning we can’t voluntarily control them.
Then we have the iris pigment epithelium which is only 2 cell layers thick and contains a
lot of melanin.
In fact, it’s completely saturated with melanin in almost everyone so it doesn’t
really play a big role in eye color.
Instead it serves a protective function, by blocking stray light from reaching the retina.
Okay so back to the stromal layer.
The stromal layer has clumps of melanin which give your eyes its color.
There are two types of melanin, eumelanin which is a dark brown and pheomelanin which
is an orangey-yellow.
When there’s a lot of eumelanin and small amount of pheomelanin, the iris appears dark
brown.
When there’s high amounts of pheomelanin, the iris appears amber.
Pretty simple right?
But how do you get blue eyes when melanin is either brown or orange?
And if melanin gives our hair and skin its color, how come we don’t get blue hair or
skin.
Well first it’s important to understand how we see colors.
Light is an electromagnetic wave and its wavelength determines the color.
Shorter wavelengths on the visible spectrum look blue and longer wavelengths look red.
A blue object will only look blue because it reflects short wavelength blue light and
absorbs the other wavelengths.
So when you have a low amount of melanin, it leaves behind only the collagen fibres
in the stromal layer.
When light falls onto these collagen fibres, they scatter the shorter wavelength blue light
but let the longer wavelengths pass straight through, only to be blocked by the pigment
epithelial layer.
This causes the illusion that iris is blue.
And it’s the same reason why the sky is blue.
Sunlight has multiple wavelengths of light, but the blue light gets scattered by air particles
due to a phenomenon called Rayleigh scattering.
In the iris, it’s called Tyndall scattering, only because the collagen particles are bigger
than air particles but they’re pretty much the same mechanism.
In fact you can demonstrate Tyndall scattering by getting a bit of milk and then adding a
lot of water to it.
Milk is made up of little globules of fat suspended in water.
If you dilute it, the fat globules scatter light just like the collagen in your eye,
giving the milk a blue tinge.
Okay so know we understand how blue eyes work, the other eye colors are really simple to
figure out.
So we said lots of eumelanin gives dark brown eyes and lots of pheomelanin gives amber eyes.
When you have some but not a lot of pheomelanin, the yellowish pigment mixes with the scattered
blue light, creating green eyes.
If you add a bit of brown eumelanin to the green eyes, you get hazel eyes, though usually
hazel eyes are brown towards the center and green towards the outside.
It's also interesting to note that your eye color subtly changes with your emotions.
This is because your sympathetic nervous system, which is responsible for your flight or fight
response, causes your eye to dilate when you're scared or nervous.
And your parasympathetic nervous system causes your eye to constrict when you're resting
and digesting.
When your eye constricts or dilates, the concentration of melanin in the stroma will change, causing
subtle changes in your eye color, though you probably can't notice it.
You might have also noticed that people with albinism can have red eyes.
Remember that I said the posterior layer, the pigment epithelium, is saturated with
the maximum amount of melanin in most people.
However, in albinism, there’s no melanin so the light can pass straight through the
iris.
The red color we see is because light bounces back off the blood vessels at the back of
the eye just like when you get red eyes on flash photographs.
However, this only happens when the light comes in at a certain angle so most of the
time people with albinism will have blue eyes due to Tyndall scattering.
Then we’ve got grey eyes.
Scientists aren’t 100% sure how people get grey eyes but the main theory is that people
with grey eyes have excess collagen deposition.
The excess collagen makes the particles in the stroma bigger and, as result, all wavelengths
of light are scattered instead of just blue.
This is called Mie scattering and happens when particle size is beyond a certain threshold
for Tyndall scattering to occur.
Mie scattering is the same reason why the sky looks grey on overcast days.
The large droplets of water in the sky scatter all wavelengths of light making it look white
or grey.
It’s also interesting to note that some people can actually have multiple colours
in their eyes in a condition called heterochromia.
Complete heterochromia occurs when the two irises are of different colours, partial heterochromia
occurs when a section of the iris is different from the rest and central heterochromia occurs
when inner part is different from the outer part.
It usually happens from birth and is benign but sometimes it’s as a result of a more
serious genetic condition.
It can also develop later in life as a result of trauma or disease.
To summarise, eye colour is a spectrum which goes from blue to green to hazel to brown
with rare exceptions such as grey or red.
And all this comes from a combination of melanin pigments and scattering of blue light.
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And that's it for today, thank you.