The answer, my friends, is blowing in the tapetum lucidum

Here's an old verse you probably remember from childhood days:

Roses are red,
Violets are blue,
Sugar is sweet,
And so are you.

As valley girls everywhere used to say, gag me with a spoon.

Well, roses may be red and violets may indeed be blue (actually, I thought they were purple), but it is also true that humans are red and doggies are green.

Don't be confused, folks, I just changed horses in midstream. I'm referring to eyes in photographs:

Son of a gun.

I brought up the question a couple of posts back and a comment by our Scottish friend Adrian Ward to the effect that the eyes of some animals have a reflective layer behind the iris inspired me to do some research.

I found this:

"In dogs (and many other animals, but not people), the retina has a reflective layer behind it called the tapetum lucidum, which acts like a mirror, reflecting light at the back of the eyes. The reflective layer is what helps dogs see better at night. Light is reflected outward, giving the dog's retina a second chance to absorb the rays."

...and this:

"Short answer: In humans it is basically the red choroid plexus in the back of the eye you are seeing on a flashed photo, while it is the green-reflecting tapetum lucidum in dogs."

The red-eye effect in humans was explained nicely by Yale Scientific Magazine:

"The human eye can effectively adjust to different light conditions, but this adaptation is also what leads to the red-eye effect. The eye regulates the amount of entering light by contracting or expanding the pupil. At night, your pupils will accordingly enlarge to extract more light from their surroundings. However, this dilation also leaves your eyes unprepared for the sudden burst of light from a camera flash.

"When light from the flash enters your eyes under these conditions, your pupils are unable to contract fast enough (full constriction takes a few seconds) to prevent the light from reflecting back, off of the red blood vessels of the choroid. The choroid plexus nourishes the retina, and therefore contains ample blood supply. As a result, the camera will pick up the red reflection, resulting in the red-eye effect. Interestingly, due to their increased “dark adaptation,” children more commonly have red eyes in photographs.

"The amount of melanin, a light-absorbing pigment in the eye, also has a role in the red-eye effect. Individuals with lower levels of melanin will absorb less and thus reflect more light, explaining the high incidence of the red-eye effect in albinos.

"Though the cause of this effect is wired in the biology of the eye, some cameras can reduce red-eye by sending a few preliminary flashes before the final flash to give the pupils enough time to contract and adapt to the increased-light conditions. Another way to reduce the effect is to avoid looking directly into the camera lens, which will reduce the reflection of light. Finally, if all else fails, modern image editing software, such as Photoshop, can remove the red discoloration.

"With regard to why dogs and other animals may not have red-eyes but other colors, some animal species have a tapetum lucidum, which is situated in front of the choroid and reflects light back onto the retina thereby increasing light sensitivity. In dogs it is green. The following figure from Pets.SE shows the anatomy and the position of the tapetum in front of the choroid.

"Hence, the reason why animals with a tapetum lucidum have another eye color is that light does not reach the choroid in these species, giving rise to green-eye effects in dogs and cats."

...and finally, deep in wikipedia's article on the tapetum lucidum, in a subsection entitled "Eyeshine", I found this:

"Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the pupil appears to glow. Eyeshine can be seen in many animals, in nature and in flash photographs. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior night vision). Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink and red (emphasis mine). However, since eyeshine is a type of iridescence, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum-lucidum crystals.

"White eyeshine occurs in many fish, especially walleye; blue eyeshine occurs in many mammals such as horses; green eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in coyote, rodents, opossums and birds (emphasis mine).

"Although human eyes lack a tapetum lucidum, they still exhibit a weak reflection from the fundus, as can be seen in photography with the red-eye effect and with near-infrared eyeshine. Another effect in humans and other animals that may resemble eyeshine is leukocoria, which is a white shine indicative of abnormalities such as cataracts and cancers."

Folks, if you want any more information on this subject, I encourage you to look it up for yourself.