Why are the pupils in your eyes so dark?

THAT'S THE WHY:ARE YOUR eyes blue? Brown? Maybe green or grey? When we talk about eye colour, we refer to the coloured ring of tissue called the iris. But right at the centre of the iris lies the pupil, the hole that lets light in, and that appears dark.

Why? It’s because when light enters the eye through the pupil, most of it gets absorbed by tissues within the eye, and precious little light is reflected back to the outside world.

“When you send 1,000 photons of light into the eye, only about one photon will come back out,” explains Dr Brian Vohnsen, a Science Foundation Ireland-funded Stokes lecturer at University College Dublin’s school of physics. “That is why your pupil is dark, very little light comes out again.”

As an aside, we can get get “red eye” in photographs when a flash is used because red light is reflected out of the eye more than blue, he adds. “This is why we often have red pupils in strong flash photographs with the flash directed straight into the eye.”

Vohnsen and his advanced optical imaging group at UCD have been developing ways to make the most of even the small amount of light which gets reflected back.

They can send a safe laser beam into the eye through the pupil and scan the back of the eye, or the retina. “We scan across and a little bit of the light is reflected back out again, and that builds up the image,” explains Vohnsen.

Because they are working at a high resolution, they can image individual cells at the back of the living eye, and this should help to pick up early signs of damage in conditions such as age-related macular degeneration, he adds.

So far, the group has been testing the technology on their own eyes, but the hope is that it will be brought to clinical trials and patients will benefit from the images caught through the dark pupil.