You probably think you know the  color of your eyes pretty well.

After all, most of us see them staring  back at us in the mirror every day. And while our bodies may  change, eye color is permanent.

Until it’s not. There are a shocking number of things that  can make your baby blues not so blue anymore, or can make your brown-eyed  girl need a new nickname. Here are just a few of them.

First, let’s start by breaking  down how the iris works. Your iris is the gatekeeper  to light entering the eye. It contains the muscle to open and close the pupil  so it regulates how much light gets in there.

Irises also come in a  spectrum of spectacular colors thanks to their cellular makeup. There are two layers of the iris. Within these layers, there’s a whole bunch of very important stuff that all helps you see.

But the bits we want to focus on are the ones that have to do with eye color, the melanocytes. Melanocytes are found in both layers of the iris. Two thirds of all cells in the anterior layer, which is called the iris stroma, are melanocytes.

So this layer is doing a lot of heavy lifting in giving your eyes their hue. And melanocytes aren’t just in your eyes. They make melanin all throughout your body.

This is the same melanin that gives your skin and hair whatever color they’ve got. Melanin absorbs light, meaning it can’t be reflected back to be perceived by us. More melanin in a spot means more light is being absorbed there, and we see it as a darker color.

So in brown eyes there’s lots of melanin present and lots of light being absorbed. But lighter-colored eyes have less melanin, which means that the shorter wavelengths of light get reflected back off of collagen fibers. And then when someone else looks  at those melanin-lacking eyes, their eyes pick up on that reflected light, and that is how we see each other’s eye color.

Melanin also comes in two forms, eumelanin, which is black or dark brown, and pheomelanin, which is light brown, yellow, or red. And different combinations  of eumelanin and pheomelanin will affect the color of your eyes, kind of like mixing paint colors For example, people with green eyes have more  of that yellow-red pheomelanin. But not all eye colors are the result of melanin.

If you’ve got albinism or another condition where you have little to no melanin, it’s the blood vessels in the  retina reflecting back light that give a pink or reddish hue. There’s also a lot that can change in a complicated system like the eyeball, so let us talk about a few  ways your eye color can change! [ INTRO MUSIC ] Although babies spend about 9  months developing in the womb, when they come into the  world all fresh and squishy, they are not done cooking just yet. And it’s not just growing  bones and developing brains that change over the coming months and years.

The eye color can change, too. Many babies have lighter eyes at  birth than they will as adults. The idea is that melanocytes in those baby blues are either not finished moving into place or the cells that are there haven’t gotten around to  specializing as melanocytes yet.

Over the first few months of life, the iris is populated with  those pigment making cells and gets darker. For the most part, the eye color will settle by around six years old. But for some it doesn’t end there.

A twin study found that 10-15%  of white participants’ eyes continued to darken between age 6 and adulthood. The fact that this happens in twins shows a pretty strong link between age-changing eye color and genetics, so your darkening eyes might  even run in the family. But there’s a twist!

Not only can growing to adulthood  cause your eyes to get darker, aging all the way to your golden years can eventually make your eyes lighter. Kind of. Arcus senilis is a build up of lipids that can form around the iris and give it a light blue or gray ring.

A sort of eyeball halo if you will. This usually happen in elderly people and the new hue is usually benign. But they can sometimes be a sign  of another underlying condition.

So it’s not a bad idea to get them checked out. There are also plenty of  not-so-great eye conditions that can change your eye color. There are a few that can also lead to loss of pigment and lighter eyes.

Horner’s syndrome is caused  by damage or disruption to certain nerves in the head and neck. That comes with a whole bunch  of neurological symptoms, but it also causes your eyes  to get lighter in color. It’s super noticeable in babies, but in adults, it can be harder to notice  because it happens so slowly.

You might only spot it when you look at a photo  from 20 years ago and think, “hey, were my eyes like, way darker back then?” Likewise, Fuchs heterochromic iridocyclitis causes inflammation of structures  in the front of the eye, including the iris. And eventually, all that inflammation  comes with a lightening of the eyes. And are you proud of me for saying, “Fuchs heterochromic iridocyclitis” correctly?

Because I am.   It’s not fully understood  what causes this to happen, but studies have found that people with Fuch’s have fewer melanocytes and smaller melanin producing machinery. And finally, we have pigment dispersion syndrome. The name basically just means a loss of pigment, which causes your eyes to get lighter.

But it's the mechanism of this particular change where things get really interesting. Some researchers have proposed that pigment loss happens with this condition because the iris is just too big for the eye, which causes it to bow backwards. This backward bowing causes the tiny thread-like fibers that hold the eye's lens in place to come into contact with the iris, which they are not supposed to do.

And when that happens, those threads can literally rub the pigment off. It can also cause some other  not-great things like glaucoma. But fortunately, glaucoma treatments can help treat pigment dispersion syndrome, too.

Now instead of pigment gain or loss, the next one is kinda like DIY eye jewelry. Kayser-Fleischer rings are brown or grayish rings that can form a little ring around the iris. They’re caused by deposition  of excess copper in the cornea, which lays over the top of the iris.

These rings are common in  patients with Wilson’s disease, a genetic disorder which causes an error in copper metabolism in the body, but they can also be caused by  other problems like liver disease. On the plus side, as long as you treat the condition  that caused them to appear, Keyser-Fleischer rings should go away. Another bummer of a disease that can change eye color is cancer.

Yes, iris melanomas are a thing. They’re not super common, but they do happen. Just like a skin melanoma, iris melanoma is a cancer starting in the melanocytes.

And just as on the skin, this can present on the  iris as dark brownish spots. They can also cause you to  start seeing more floaters, and can change the shape of your pupil. Most iris melanomas will not be visible, but if new dark spots appear on your iris, it’s probably time to book an  appointment with your doctor.

And not only can a cancer  cause your eye color to change, treating cancers can cause it too. There’s a rare type of eye  cancer found in young children called retinoblastoma that is super aggressive. But there’s a new treatment for retinoblastoma which involves injecting chemotherapy drugs into the vitreous humor, which is the jelly-like substance between your lens and retina, of the affected eye.

This sounds terrible, but like… we're getting the cancer. And while that treatment  seems to work pretty well, there are a couple case studies that suggest the treatment also resulted in that eye getting lighter, giving those kids permanent heterochromia and a hell of a story to tell. We’ve got more cool science to share.

But first, let me tell you about another way to learn cool science: Brilliant. With Brilliant, you don’t even need to limit yourself to science. You can choose from thousands of lessons in computer science and math too!

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With Brilliant, you can train models on real data  and generate all kinds of products, like a poem or cover letter. And you can learn how the data your model trains on changes the product by comparing what comes from a model trained on Taylor Swift versus one trained on a cookbook. While you’re playing with those variables, you’ll build an understanding  of large language models from the ground up.

And since you learn this stuff  through hands-on problem solving, you’re more likely to actually remember it. To get started, head to Brilliant.org/SciShow or the link in the description down below. That link also gives you 20% off an  annual premium Brilliant subscription.

And you’ll get your first 30 days for free! And thank you to Brilliant for  supporting this SciShow video! Now we’ve talked about a lot of weird diseases that can change your eye color, but there are some medications  that can do this too.

One of the most common is a medication group called prostaglandins, which are used to treat glaucoma. Glaucoma is just a blanket  term for a group of conditions that cause increased pressure in your eye, and over time, damage to the optic nerve. And prostaglandins are great  at lowering that pressure, so they are great for treating glaucoma.

And they are also known for a weird side effect – darkening people’s irises. One study found that the  glaucoma medication latanoprost caused between 5 to 23% of patients’ eyes to get darker after 12 months of treatment. It’s probably still a good idea to take these medications either way.

Just don’t be surprised if  you notice a new eye shade in your reflection afterwards. We’ve talked about dark flecks  and white rings on the iris, but how about patchwork eye color? Vitiligo is an autoimmune disorder where your immune system turns on your melanocytes and destroys them.

It’s usually more apparent when  it’s happening in the skin, since it causes large patches  of skin to become much lighter than they were before. But it can happen to melanocytes in your eyes too, which can look like patchy blue spots in the iris, or more widespread loss of pigment. This specific presentation of vitiligo is often associated with  specific eye related syndromes like Vogt-Koyanagi-Harada syndrome and Alezzandrini's syndrome but can also happen in the more  general, full body vitiligo.

And this is probably the most temporary eye color  swapping method on our list – but your eye color can look really different depending on your emotional state. Novelists might be taking some artistic liberty when they describe eyes becoming  “dark and stormy” in anger but there really is something there. Not all eye color changes come down to the iris.

The size of the pupil plays a role too. Your pupil dilates differently when you are processing different emotions. One study found that fear can cause the pupil to dilate more drastically than feelings of happiness or neutrality, which would temporarily make your eyes look a different color as  less of your iris is visible.

And of course this isn't  really an eye color change, but it is the illusion of one. Another illusory eye color change can come from trauma to the brain or eye. A nasty knock on the head can cause your pupil to get permanently stuck at full dilation.

And, like we mentioned before, greater pupil size equals less visible iris and a whole different looking eye color. This can happen in both eyes  but usually it’s just one. And if this ever happens to you, you're in good company, since a permanently blown pupil was also the cause of David Bowie’s famous mismatched eyes.

Whether it also makes you a rock legend… less clear. Everything we’ve talked about so far that can change your eye color comes down mostly to a roll of the dice. But what if you could choose your own adventure and actually select your eye color.

It sounds like science fiction but for some, this has become a reality. Congenital Aniridia is a rare genetic eye disorder where you are born without a  part of or all of your iris. As with trauma to the eye, this means that your pupil is  larger and your eyes look darker.

Without the pupil control, this of course leaves people  with sensitivity to light and even lead to vision loss. But there’s a procedure  that can help correct this - an iris implant! They are not FDA approved yet, but doctors can get exemptions to use them on compassionate grounds in the US, and they are available in some other countries .

And the caution to approve  them for wider scale use might be warranted. Because although iris implants might improve vision and  reduce glare in individuals with Aniridia , they might also cause some nasty complications like glaucoma. These prostheses are still pretty new, so it’s up to each individual and their doctor to weigh the risks and benefits.

So if you do have a condition where having a new iris could  really help, technically, you could go for any color you want. Maybe throw some stripes on  there and get funky with it. So now you know!

There’s a lot that can happen  to change your eye color. And while it may be jarring to see such a change, no matter what color they are, we think that beauty is in  the eyes of the beholder. [outro]