Science in School: Glowing Eyes? – You (and your Dog) are not Alone


Aarna Pal-Yadav

How many times have you taken a picture at night and found your eyes glowing a devilish red? In the meantime, your dog’s eyes are a beautiful shade of green or blue. No, you’re not possessed and, yes, there is a cure.

The science behind glowing eyes is fairly simple. Human eyes are made of the sclera, pupil, iris, cornea, lens, and retina. The sclera is the white part of your eye that protects it from too much light because the white color reflects excess light. The pupil is the black dot which allows certain amounts of light in. The iris is the colorful part of our eyes which expands or contracts the pupil to allow more or less light in. The lens focuses the light which enters the eye onto the retina. The retina is a light-sensitive blood-colored lining the back of our eye which turns light into electrical impulses that turn into what we see. When we take pictures at night, we usually turn the flash on. That flash comes in through our pupils, and since the flash is so sudden, the iris cannot contract the pupil in time to limit the amount of light coming in. The lens focuses the bright light onto our retina, but that’s when something interesting happens. The retina does turn some of the light into impulses (we know that because we can see the flash), but the rest is reflected back at the camera. Since the retina is a mass of blood cells, the light reflected takes on a red color. That’s why pictures with flash make human eyes red. But what about dogs?

Dogs have another part of their eye called the tapetum lucidum. The tapetum lucidum sits behind the dog’s retina and its main purpose is to reflect light to the retina. This is because dogs were originally nocturnal and needed good night vision. Essentially, the tapetum lucidum is a mirror behind the retina so whatever light isn’t caught the first time has a second chance. That means when bright light hits a dog’s eyes, the light bounces off the tapetum lucidum and back to the camera. The tapetum lucidum is also much more reflective than the retina so any redness is washed away. That is also why a single flash isn’t the only thing that makes dogs’ eyes glow; flashlights also make their eyes glow for a sustained period because the tapetum lucidum is much more reflective. The blue or green glow is usually caused by the dog’s eye color and the chemical composition of the tapetum lucidum.

So how do you fix red-eye? Don’t worry, you don’t have to stop taking pictures at night or stop using flash. In the dark, human eyes dilate, or expand, to let in more light so we can see better; in the light, your eyes contract to limit the amount of light entering. But it takes time for the pupil to dilate or contract. If it’s dark outside, your eyes dilate to let in more light, but when you take picture with a single flash, you are suddenly bombarding your eyes with a huge amount of light – your pupils don’t have time to contract and limit the huge amount of light entering, so all of that light is reflected off the retina. Many phones are starting to use a double flash. The first one allows the human’s eyes to contract, limiting the amount of light that reaches the retina in the second flash. So, next time you’re on a trip and want to take some night-time pictures, turn on double-flash and keep your eyes their natural color.