Why Do Cat Eyes Glow in the Dark?
That eerie glow you see when you shine a flashlight at a cat’s eyes in the dark - it’s just plain creepy. Or is it creepy cool?
The first time I noticed this, I was a bit startled, but lately I've been wondering about the science behind it. Before we delve into why they seem to glow in the darkness of night though, it's important to understand what makes cat’s eyes so unique in general:
The Structure of a Cat's Eyes
Cat’s eyes have certain advantages that ours don’t - a higher concentration of rods within their eyes. Rods are sensitive to low-light conditions so with more rods, comes a better ability to see in the dark. In fact, expert say “cats eyes are able to function in approximately one-sixth of the light needed for human vision!”
Part of this is also due to the fact that a cat’s eyes are normally larger than a human's. This allows their pupils to dilate wider and take in more light than ours could - another important factor when it comes to night vision. Unfortunately for them though, this advantage at night can become a disadvantage during the day. When it’s a sunny, the ability to contract your pupils smaller becomes desirable and so cats can't see as well as they would in the dark.
To help their night-eyes cope in the bright light of day, cats must use two ciliary muscles, while humans only need one. This allows their pupil to dial itself down to a tiny slit during the daytime.
The "Glow in the Dark" Effect
The secret behind the nighttime glow in the eyes is the "tapetum lucidum."
This part of the eye is positioned in the back of the eye, behind the retina and it functions like a mirror. It reflects light back onto light sensors cells in the retina. Basically, it's a way of absorbing the brightness from any light sources at night to help them see better, but it also creates and interesting effect when exposed to bright lights.
When any sort of light shines into a cats eye at night, it causes this night-time glow. As you may have noticed camping or driving at night on a country road, this effect is not unique to cats alone, but is true of many nocturnal animals. Car headlights, street lamps, and even your flashlight will cause their eyes to "glow" in the night!