The Science Of How Eyes Glow In The Dark

When we see animals' eyes in photos, nature films, flashlight beams, and headlights we see them shining back at us — but human eyes only glow bright red in photos. What causes the shine? What causes the color?

The glow of red human eyes is a little like the glow of the moon. It's the reflected light of a far greater light source. The flash of a camera travels through the pupil and hits the back of the eye – the retina. The retina reflects the light back towards the camera, but it does so just a little differently than it came in. Like most of the insides of our bodies, the retina is an icky red mess, with blood vessels criss-crossing it. Those blood vessels color the light that shines back red. We literally dye the light red with our blood.

The Science Of How Eyes Glow In The Dark

But what about animal eyes? Dogs, cats, and deer have blood, and their eyes don't look like vampire eyes. Their eyes glow because of a layer called the tapetum lucidum just behind the retina. This layer reflects light because that's exactly what it's meant to do. Cats, dogs, deer, and other nocturnal animals have good night vision because whatever the photo-receptor cells in their retina doesn't catch, hits the tapetum lucidum and takes a second pass at the retina again. They get more photons from the same scene.

Since the tapetum lucidum is a great deal more reflective than the retina, the redness is washed away by whatever materials make up the reflective layer. Although pigment from the retina and from pupil can influence the color of the shine, most of it comes from the slight differences in levels of elements like zinc in the reflective layer.

It's the lack of this layer that causes humans to have shining eyes only in photos, and not in flashlight beams. The relatively weak retina can only shine through during the powerful, and brief, flash of a camera. The flash provides a lot of light, and it's light that's usually directed at a person's face. The camera also gives off this light only for a brief second. Some cameras eliminate red eye by making two flashes. Flashes are used when it's dark, and darkness dilates the pupil, letting the retina take in as much light as possible. When the first flash goes off, the pupil constricts, and doesn't let enough light in to allow the retina to make someone look like a ghoul.

Top Image: Prime Minister of the Russian Federation.

[Via Red Eyes In Photographs, Why Do Animals' Eyes Glow in the Dark, Why Do Dogs and Cats' Eyes Shine in the Dark]