(Intro)

You know that thing where your eyes glow bright red in photos? It’s called the red-eye effect.

But it doesn’t mean you can secretly fire laser beams from your eyes or anything. Instead, you can blame the bright flash of a camera, and how your eye adapts to changes in light. Your eyes are basically fluid-filled orbs that can detect light and send messages to your brain, so you can see images.

Light passes through a thin membrane called the cornea, then a hole called the pupil, and then a lens – so it can be focused, and absorbed by photoreceptor cells in the back of your eye. Your pupil is surrounded by this muscle-filled structure called the iris, which is the colored part of your eye. You don’t consciously move these muscles, but they control how big your pupil is, and how much light you let in.

When it’s dark, your iris makes the pupil wider to let more light into your eye. But when there’s bright light, your iris makes the pupil smaller. So, the red-eye effect usually happens in a dark environment, when your pupils are really wide.

Normally, all the light that enters your eye is absorbed by photoreceptors, or by a pigment called melanin in a tissue layer at the back of your eye – the same pigment that influences the color of your iris, hair, and skin. And since all the light’s absorbed, your pupils look black. But if there’s suddenly a bright camera flash, all that light floods into your eyes before your iris muscles have time to contract, some of it might reflect off the blood vessels in the back of your eye, and it shows up as a glowing red light to ruin that group photo you were trying to take. To fix this, some cameras make a couple quick flashes of light before the actual flash when the photo is taken, so that your iris muscles start contracting, and let in less light. You could also try brightening the room, so your pupils aren’t as wide in the first place, or avoid looking directly at the camera lens.

The red-eye effect is pesky, but it isn’t always bad. It can actually be a handy tool for diagnosing eye problems. If someone was looking directly at the camera lens in dim lighting, and they have a glowing white or yellowish eye, there might be an infection, some cancerous cells, or those blood vessels might be twisted or leaking.

Sometimes photos have one glowing red eye, which might mean someone’s eyes aren’t looking in the exact same direction. Or, it could mean there’s different amounts of melanin in the back of each eye, so they’re different colors. So mostly the red-eye effect is just annoying, but you might want to keep an eye on it.

Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you’d like to submit questions to be answered, or get some videos a few days early, go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!