Grab a partner and look into his or her eye "“ or stare deeply into it, if appropriate "“ in a normally lit room. The quick and dirty version of how you're able to view each other is this: Light enters the eye through the cornea, the clear outer dome, and goes through the pupil. Then it travels to the cornea, which focuses it on the lens. The lens further focuses the light and spreads it across the retina. The retina receives the light and transmits signals via the optic nerve to the brain, which interprets the image.
As light enters the eye, some of gets reflected back, but the amount of light in most situations is so small, you wouldn't even know it. Right now, your partner's pupils look black and everything's normal. Now grab a camera and take a picture of your partner with the flash on. There's that demonic red eye.
Here's what happened: When you took the picture, the camera flash sent a lot of light into the eye in a very short time, the light reflected off the back of the eye and out through the pupil and, because the camera lens is close to the flash and able to capture images very quickly, it caught the light reflecting back out.
So why is that light red?
Because the fundus, the interior surface of the eye that includes the retina, is loaded with melanin, a pigment that gives it a brownish-reddish color. Was that anti-climactic? Sorry.
Red eye is fairly easy to curb by using the "red eye reduction" setting found on most digital camera flashes. This setting causes the flash to go off once before the picture is taken, which causes the subject's pupils to contract and let less light in and out, and then another time to take the picture. Cameras with a flash farther away from the lens also reduce red eye because the flash hits the subject at a different angle than lens captures it.
Of course, red eye isn't all bad. The same mechanics of light reflection that ruin photos also allow doctors a non-invasive way to see inside the eye. Hermann von Helmholtz, a German physician, discovered in the 19th century that he could examine the retina by holding a bright light near his eye and shining it into patients' pupils.
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