They’re one of the first things we notice about someone.
But if you’re a sucker for big blue eyes then you could be in for a shock – as they’re not really blue at all, according to science.
Experts have revealed that the blue hue of an iris doesn’t come from pigment but from the scattering of light.
This phenomenon, known as the Tyndall effect, is echoed in the way both the sky and sea appear blue – but are in fact clear.
Dr Davinia Beaver, from Bond University in Australia, explained that melanin – a substance in the body responsible for the colour of our skin and hair – is to blame.
‘Brown eyes contain a high concentration of melanin, which absorbs light and creates their darker appearance,’ she wrote on The Conversation. ‘Blue eyes contain very little melanin.
‘In blue eyes, the shorter wavelengths of light – such as blue – are scattered more effectively than longer wavelengths like red or yellow.
‘Due to the low concentration of melanin, less light is absorbed, allowing the scattered blue light to dominate what we perceive. This blue hue results not from pigment but from the way light interacts with the eye’s structure.’
Experts have revealed that the blue hue of an iris doesn’t come from pigment but from the scattering of light (stock image)
Green eyes, on the other hand, are rare because they are the result of a genetic quirk that lowers levels of melanin – but not as low as in blue eyes.
Hazel eyes are even more complex – as uneven melanin distribution in the iris creates a ‘mosaic’ of colour that can shift depending on the light.
Despite a decades–long assumption that eye colour is controlled by a single gene, experts have recently learned that many genes determine this trait.
‘This explains why children in the same family can have dramatically different eye colours, and why two blue–eyed parents can sometimes have a child with green or even light brown eyes,’ Dr Beaver said.
To explain why many babies – especially with European ancestry – are born with blue or grey eyes, Dr Beaver said melanin levels are still low at this age.
As pigment gradually builds up over the first few years of life, those blue eyes may shift to green or brown.
By adulthood, eye colour tends to be more stable but can be slightly influenced by light or even the shade of the clothes you’re wearing.
Permanent shifts are rare but can occur as people age, or in response to certain medical conditions.
Musician David Bowie’s eyes appeared as different colours because of a permanently dilated pupil after an accident
Some celebrities, including Kate Bosworth and Mila Kunis, have a rare condition called heterochromia, where one eye is a different colour to the other.
It can be genetic, the result of injury or linked to specific health conditions, Dr Beaver explained.
Musician David Bowie’s eyes appeared as different colours because of a permanently dilated pupil after an accident, giving the illusion of heterochromia.
Blue is now the most common eye colour in the UK, figures suggest, while brown eyes are the most common eye colour in the world.
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