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Speaker 1: Welcome to Brainstuff from house Stuff Works dot com where

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Speaker 1: smart happens. Hi. I'm Marshall Brain with today's question, why

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Speaker 1: do people have red eyes? And flash photographs? We've all

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Speaker 1: seen red eye where the people in the picture have

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Speaker 1: spooky red eyes. These are photos taken at night with

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Speaker 1: a flash. Where do the red eyes come from? The

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Speaker 1: red color comes from light that reflects off of the

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Speaker 1: retina's in our eyes. In many animals, including dogs, cats,

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Speaker 1: and deer, the retina has a special reflective layer called

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Speaker 1: the tapatum lucid um that acts almost like a mirror

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Speaker 1: at the backs of their eyes. The idea is that

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Speaker 1: their retina's will reflect light back onto the scene, giving

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Speaker 1: these animals better night vision. If you shine a flashlight

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Speaker 1: or headlights into their eyes at night, their eyes shine

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Speaker 1: back with bright white light. Humans don't have this reflective

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Speaker 1: layer in our retinas. If you shine a flashlight in

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Speaker 1: a person's eyes at night, you don't see any sort

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Speaker 1: of reflection. The flash on a camera is bright enough, however,

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Speaker 1: to cause a reflection off the human retina. What you

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Speaker 1: see is the red color from the blood vessels nourishing

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Speaker 1: the eye. Many cameras have a red eye reduction feature.

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Speaker 1: In these cameras, the flash goes off twice, once right

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Speaker 1: before the picture is taken, and then again actually to

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Speaker 1: take the picture. The first flash causes people's pupils to contract,

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Speaker 1: reducing red eye significantly. Another trick is to turn on

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Speaker 1: all the lights in the room, which also contracts the pupil.

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Speaker 1: Another way to reduce or eliminate red eye in pictures

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Speaker 1: is to move the flash away from the lens. On

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Speaker 1: most small cameras, the flash is only an inch or

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Speaker 1: two away from the lens, so the reflection from the

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Speaker 1: flash comes right back into the lens and shows up

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Speaker 1: in the photo. If you can detach the flash and

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Speaker 1: hold it several feet away from the lens, that helps

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Speaker 1: a lot. You can also try bouncing the flash off

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Speaker 1: the ceiling if that's an option. Do you have any

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Speaker 1: ideas or suggestions for this podcast? If so, please send

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Speaker 1: me an email at podcast at how stuff works dot com.

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Speaker 1: For more on this and thousands of other topics, go

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Speaker 1: to how stuff works dot com.