 |
| Wolves and many nocturnal animals have glowing eyes under the light. (Photo: Inspirationline) |
The human eye also reflects light, as every photographer knows… but not very well. In some photos, our eyes also shine red. However, it is not a reflective mirror. This occurs because the light from the camera reflects off the red blood vessels and the red muscle behind the retina.
The bright eyes characteristic of wolves, North American raccoons, crocodiles, and many other species, arise from the “tapetum lucidum” – a layer of cells that acts like a mirror located behind the retina. This structure is present in most nocturnal animals, serving as a light amplification system. Typically, the retina absorbs some of the light that enters the eye, but allows some of it to pass through. The tapetum lucidum mirror reflects the light that passes through back to the retina, giving the animal a chance to “see” it a second time.
It is this returned light that has inspired countless horror stories that frighten superstitious people.
So what about human night vision? Cats adapt better than we do in low light conditions. They have a wider cornea and pupil than humans, allowing them to capture more light in dim conditions. They also possess a tapetum that helps reflect light back into their eyes. In this way, their retinas have two opportunities to capture the same photon of light. The tapetum in a cat’s eye reflects light 130 times, which is stronger than human eyes. This is why cat eyes glow brightly in the dark in photographs.
You could argue that humans are not nocturnal animals, but we certainly have the best daytime vision. In this regard, we fall far short of eagles. Eagle eyes are similar in size and weight to human eyes. However, their shape is quite different. The back of their eyes is flatter and wider than ours, giving them a much larger field of view.
The back of the human eye has a special area on the retina called the fovea – where many light-sensitive cells are concentrated. The human fovea has about 200,000 cone cells per millimeter, a number so large it’s hard to imagine. But this is nothing compared to eagles. Their fovea has about 1 million cone cells per millimeter. This allows eagles to see a mouse from a distance of 1.6 kilometers, which is beyond human capability.
Thuan An
