Halloween is approaching, and a whiff of ghoulish menace is squatting casually in the darkness of London’s evenings. Ghostly figures, silhouettes of witches and jagged glowing teeth loom over me as I walk home, but it’s their eyes that I notice most: bright orbs watching me through a pupil that is often a dark vertical slit. That slit seems like a warning, a signal that whatever is behind it is out to get you. But owners of slit-shaped pupils aren’t rare in our world: Along with crocodiles and vipers, our cute fluffy pet...

Physicist Helen Czerski explores the complex science behind familiar phenomena. Read more columns here.

Halloween is approaching, and a whiff of ghoulish menace is squatting casually in the darkness of London’s evenings. Ghostly figures, silhouettes of witches and jagged glowing teeth loom over me as I walk home, but it’s their eyes that I notice most: bright orbs watching me through a pupil that is often a dark vertical slit. That slit seems like a warning, a signal that whatever is behind it is out to get you. But owners of slit-shaped pupils aren’t rare in our world: Along with crocodiles and vipers, our cute fluffy pet cats all have them. So why are some eyes like this, and are they really the ones to be afraid of?

The pupil of an eye serves a very specific purpose: It’s the gatekeeper. Light must pass through it to reach the back of the eye where the light-sensitive cells are. The pupil is needed because when it comes to vision, more light isn’t always better. Our eyes have an adjustable lens to focus the light, but any lens can only focus light from one distance at a time, leaving the rest blurry. Shrinking the pupil increases the distance over which the image is in focus—what photographers call the “depth of field”—which in turn reduces blur. This is fine when enough light can still come through the very small pupil, but it doesn’t work in the dark, and so there’s always a trade-off. An animal’s pupil can tell you a lot about that species, because it tells you which trade-offs are most advantageous to them.

Across the animal kingdom, vertical slit-shaped pupils overwhelmingly belong to low-to-the-ground, nighttime ambush predators who also need to function during the day. The muscle structure needed to open and close a slit-shaped pupil is simpler, and so in the brightest light, a cat can narrow its slits to the tiniest sliver, and then open them up in the dark until they’re round. A domestic cat can change the area of its pupils (and therefore the light it lets in) by a factor of 135, while we humans can only change ours by a factor of 15. So cats can see better than us in the dark, betraying their evolutionary origins as nighttime hunters millennia before canned cat food was invented. But the more interesting trade-off is in the slit shape.

“An ambush predator needs to be absolutely perfect at distance measurements, because it only has one chance to pounce.”

A cat—like its fellow nighttime predators—effectively has a narrow pupil in the horizontal direction but a wide pupil in the vertical direction. So anything along the horizontal plane is in beautiful sharp focus, and the image is more blurred in the vertical direction. But at the right focal length, the image will be perfectly in focus in both directions, and that gives the cat an extra way of judging distance.

An ambush predator needs to be absolutely perfect at distance measurements, because it only has one chance to pounce. The slit gives the cat a second way of doing this (the other is the stereo images from both eyes, which is how humans judge distance); the two complement each other to produce pinpoint accuracy.

Out on the streets at Halloween, this doesn’t sound reassuring. The mysterious creatures staring at me from the seasonal decorations with slit-shaped eyes are definitely nocturnal ambush predators, capable of pinpointing my position extremely accurately. But since their slits are closed up and narrow rather than wide open and round, they’re cutting out most of the light from the street. And that means they almost certainly can’t see me in the dark. So I can skip through the Halloween zoo unseen, safe because physics is hiding me.