How many times have you seen the glow of your cat or dogs eyes in the beam of a flashlight or the eerie glow of an animal's eyes caught in the headlights of a car on a dark night?
Have you ever wondered why human eyes don't reflect the same way in the dark? Well, to answer these questions, let first take a closer look (pun intended) at the anatomy of the eye.
Although it is small in size the eye is one of the most intriguing and complex organs in our body and functions to provide us with the most important of our five senses — sight!
Our eyes function somewhat like cameras. The cornea is the transparent front part of the eye that protects the iris, pupil, and anterior chamber, and provide the majority of the eye's optical power. Vision occurs when light enters our eye through the pupil (the round, dark center of the eye) which opens and closes like the shutter on a camera and controls the amount of light that comes in.
When light enters the eye it becomes focused on the sensory membrane that lines the eye, known as the retina. The retina serves as a film, collecting images formed by the eye's lens. It eventually converts them into signals that are sent to the brain via our optic nerve where they are processed and interpreted.
The retina contains millions of tiny photoreceptors called rods and cones. Our rods are extremely sensitive to light and respond well in dim light and dark conditions. Rods are the cells that allow people and many animals to see in the night or in low light conditions. Unlike cones, they are unable to distinguish between colours, and have low have very low visual acuity.
Cone cells, function best in bright light and are essential for being able to resolve fine detail. Different cone cells are sensitive to various wavelengths of light to allow colour to be seen.
Alright. Now that we know how the eye works, why is it that animal eyes "glow" in the night? Scientists refer to this phenomenon as "eye shine" or retroreflection. Retroreflection depends on the tapetum lucidum, also known "bright-carpet"; a mirror-like, reflective tissue found behind the retina of an animal's eye.
Essentially, light rays that enter the eye are returned in the direction from which they came and the tapetum lucidum gives the eye a second chance to absorb as much light reflected by the retina. The light that is not absorbed is reemitted and gives rise to an eerie glow or "eye shine". Unlike animals, humans lack this reflective layer, so when bright lights hit our eyes, like the light from a flashlight, we don't see any sort of reflection.
Did you know? That in darkness cats are capable of seeing clearly in approximately only one-sixth the light needed for human vision? Amazing!!! This whole process allows animal to have sharp vision at low light intensities, especially night.
Different species have various colors of eye shine or different eye shine ID's. The color of the tapetum lucidum produces a blue, green, yellow or white effect. For instance, a rabbit's eye shine is red, but that of a deer or a cat is silvery white or yellow, while a black Labrador retriever usually haves a green tapetal reflection.
Tapetal color also varies to some degree with the color of an animal's coat. Several cat and dog breeds (e.g. blue point Siamese cats and red Siberian Huskies) are known as color dilute, and lack tapetal pigment and therefore exhibit a red reflex similar to human red eye. Animal eye-shine ID is an important clue for hunters as well as naturalists.
So next time you see a pair of glowing eyes staring you in the face, you'll know it's not some ghoulish monster coming to get you but rather a local animal with some well designed night-time eyes who's just trying to get a better look at you!
CurioCity would like to thank Kim Bridger for her expertise and help in answering this question. Kim grew up in Twillingate Newfoundland, but is currently working in St. John's Newfoundland. She has an Honour's and a Master's degree in Wildlife Parasitology. Kim loves the outdoors, cycling, reading, and a good cup of coffee!!!