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Thursday 17 June 2021

Three Things I Don’t Know (Part III): Eyes

So, I asked myself, what unanswered scientific questions do I have, and are there answers out there for me? I had a think. And I came up with a list of three questions – and did my research. So here is the first of my three “Things I don’t/didn’t know” – let’s find out whether there’s an answer!

Why were my baby’s eyes indigo at birth?

Baby with dark blue eyes. Wiki Commons.
I’ve never heard of purple eyes before, but I know that the colour you’re born with can change. So, you can imagine when I looked into my baby daughter’s face and saw her eyes were a deep, dark purple that I was surprised – but I accepted it would be temporary. And temporary it was. Over the first year of her life, the time scientists say it takes eye colour to settle down, they have lightened to a medium blue, with that reminiscent darker blue round the edges.

I don’t have very clear pictures of the first month or so of her life: she kept her eyes mostly shut, and those I do have simply show their darkness, but me and her father remember that deep indigo colour – a bluish, purplish darkness, which looked indigo both under the artificial lights of the midwife unit, and under natural light from the window at home.

I turned to the internet…
Can baby’s eyes be indigo? I wondered. All the results were about “indigo children”. So I looked up purple, violet. And I found some interesting things.

Anatomy of the eye. Wiki Commons
The first interesting thing is that there is no such thing as blue eyes, not really. Eye colour depends on two factors: the amount of brown melanin (pigment) in your eyes, and Tyndall scattering – which is just an eye-specific term for Rayleigh scattering of light. People with light coloured eyes, like blue, have less melanin and lots of scattering off the fluids and tissues in their eyes. Albinos have almost no melanin, and the red colour appears when light bounces off red blood vessels at the back of the eye. This is also why it can look like you have red eyes if an intense light (like a camera flash) is shone in them. And this also means violet-coloured eyes are possible: when people are nearly albino, they can get some of that red effect mixed in with a little blue from the small amount of melanin that is present, and thus their eyes appear violet. Why there’s no direct scattering of violet light (380-450 nm) is unclear, but if there were, more people would have pale, violet eyes.

Elizabeth Taylor. Wiki Commons
But this isn’t the sort of purple my baby’s eyes were: hers were dark. Very dark. Famous actress Elizabeth Taylor was commonly thought to have purple eyes. Very dark in blue colour, under some lighting they appeared to be tinted purple. I’m still not entirely clear why this is, but some version of light scattering is likely to be to blame – both for Elizabeth Taylor and my daughter’s eyes at birth.

Another interesting thing I found is that scientists can’t agree how long it takes for a baby’s eye colour to settle down. It clearly starts changing fast and slows down, but whilst most agree that at 1 year, you pretty much have your colour, subtler changes can mean it keeps changing – up to age 2, 3, 5 or 6... depending on who you ask about it. This is much more common and more noticeable in white babies because they are more likely to have lighter coloured eyes, and is due to changes in the amount of melanin produce after they enter the world and get exposed to light.

Light should, of course, stimulate the production of melanin, making your baby’s eyes darken – which is the only direction some sources can claim it goes. However, my own experience and a plethora of mummy bloggers and forum posters show that it isn’t. Some baby’s eyes lighten. But I haven’t yet been able to find out why they’d produce less melanin. If you can find papers or know anyone working on the topic, leave a comment or Tweet us @TWeDK to share the information!

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