Welcome to Brainstuff from house Stuff Works dot com where smart happens. Hi. I'm Marshall Brain with today's question, why do people have red eyes? And flash photographs? We've all seen red eye where the people in the picture have spooky red eyes. These are photos taken at night with a flash. Where do the red eyes come from? The red color comes from light that reflects off of the
retina's in our eyes. In many animals, including dogs, cats, and deer, the retina has a special reflective layer called the tapatum lucid um that acts almost like a mirror at the backs of their eyes. The idea is that their retina's will reflect light back onto the scene, giving these animals better night vision. If you shine a flashlight or headlights into their eyes at night, their eyes shine back with bright white light. Humans don't have this reflective
layer in our retinas. If you shine a flashlight in a person's eyes at night, you don't see any sort of reflection. The flash on a camera is bright enough, however, to cause a reflection off the human retina. What you see is the red color from the blood vessels nourishing the eye. Many cameras have a red eye reduction feature. In these cameras, the flash goes off twice, once right before the picture is taken, and then again actually to
take the picture. The first flash causes people's pupils to contract, reducing red eye significantly. Another trick is to turn on all the lights in the room, which also contracts the pupil. Another way to reduce or eliminate red eye in pictures is to move the flash away from the lens. On most small cameras, the flash is only an inch or two away from the lens, so the reflection from the flash comes right back into the lens and shows up
in the photo. If you can detach the flash and hold it several feet away from the lens, that helps a lot. You can also try bouncing the flash off the ceiling if that's an option. Do you have any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com.