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Why do camera flashes make your eyes turn red?
Category: Biology
Published: May 1, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Camera flashes do not make your eyes turn red. The inside of your eyes are always red. The bright light of the camera flash just makes the color more obvious. Your eye is essentially a hollow ball filled with clear fluid. The hole at the front of your eye, the pupil, lets light into the hollow space inside the eye. The light passes through this space and then strikes the inner back surface of the eye, known as the retina. The retina is packed with cells that detect the light, change it to electrical signals, and pass the signals on to the brain where the pattern of light is experienced as a visual image. The approximately 100 million light-sensitive cells on the human retina give us an amazing visual resolution, but they also require a prodigious amount of blood to keep them going. This blood is what gives the retina its red color.
Whenever you look at someone's pupil, you are in fact looking through a hole and at their retina. Because the cavity of the eye acts like a dark cave that absorbs most of the light, the red color of the retina usually looks black under normal lighting conditions. But if the light gets bright enough, the redness of the retina becomes more obvious. The flash of light from a camera is bright enough and angled properly to make the eye's redness more apparent. The result is the all-too-familiar red-eye in photographs. But the camera's flash is doing nothing more special than providing extra light. In fact, you can see the redness of the human retina without a flash or camera. Place a friend close to a regular light bulb such that he is facing the light blub. Now place yourself between the bulb and your friend such that the top of your shadow sits just below his eyes. Now look at his eyes and his pupils will be bright red. For this to work right, the light must be bright and localized (such as a single 100W incandescent light bulb; not a flood light, a shaded light, or a collection of bulbs), other lights must be turned off or blocked (including sunlight), and your friend's pupils need to be widely dilated. You can get his pupils to dilate by turning all lights off for a minute or having him close and cover his eyes for a minute. Upon opening his eyes with the light on, there will be a brief moment before his pupils constrict when you can best see the red eye. If you don't see a very dark, crisp shadow of your head just below his eyes, then the lighting configuration has not been set up properly.