What if you could see in the dark?

Turn off the light at night. Can you see your toys? Only a little!

A cat can see much better in dim light. What if you could see in the dark too?

Switch off the lights at bedtime and your room fades into gray shapes. After a while you can make out a little more, but colors are gone and details are fuzzy.

Yet a cat can prowl around the same room with ease. So what if you could see in the dark? Would you need cat eyes, a special camera, or something else? Let's follow the clues: how cats see in dim light, how cameras see light we can't, and how warm bodies glow with invisible light.

'Seeing in the dark' can mean three different things: seeing with very little visible light, detecting light outside the visible range, or detecting heat. Each depends on different physics and biology.

This thought experiment explores all three: the adaptations that let cats see in dim light, cameras that record light our eyes cannot, and the infrared radiation emitted by warm objects. Measured facts come from biology and NASA sources; what it would be like for a person to have these abilities is labeled as an idea.

Your eyes have two kinds of light helpers. Some help you see colors in bright light.

Others help you see in dim light. But they only see gray, not colors!

At the back of your eye is the retina, where special cells turn light into signals for your brain. You have two kinds. Cones work in bright light and let you see colors. Rods are much more sensitive to light, so they work in dim light, but they only see shades of gray.

That's why colors disappear at night. And here's a trick from the OpenStax biology textbook: a faint star is often easier to see if you look slightly to one side of it, because the rods are at the edges of your retina.

OpenStax explains that rods are strongly photosensitive, lie toward the outer edges of the retina and serve peripheral and nighttime vision, detecting only shades of gray. Cones are weakly photosensitive, respond in bright light and mediate color; in humans, cones far outnumber rods in the fovea.

This arrangement explains averted vision: a dim star is better seen with peripheral vision, where rods operate, than by looking straight at it with the cone-rich fovea. Human night vision therefore trades color and sharpness for sensitivity.

Pretend you have cat eyes. Your room at night looks much brighter!

You might find your teddy bear without turning on the light.

If you had eyes like a cat's, you could see well in light only about one sixth as bright as you need now, according to Britannica. A dim room at night might look much clearer to you than it does today.

But cats pay a price: Britannica notes that their color vision is not highly developed. So with cat eyes, you might see better in the dark but enjoy fewer colors in the daytime. That trade is an idea about how it might feel.

Britannica reports that domestic cats, crepuscular hunters active at dawn and dusk, need only about one sixth of the illumination humans require to see well. Their pupils expand widely in dim light and contract to slits in bright light, and the tapetum lucidum boosts the light available to the retina.

Britannica also notes that cats' color perception is not highly developed, likely because detecting prey movement matters more for survival. A human with cat-like eyes might therefore gain sensitivity and lose some color; how that would feel is speculative.

A cat has a shiny layer at the back of each eye. It works like a mirror.

Light bounces back through the eye, so the cat gets a second look! That's why cat eyes shine in a flashlight beam.

Behind a cat's retina is a shiny layer called the tapetum lucidum (say it: tuh-PEE-tum LOO-sih-dum). Light that passes through the retina hits this layer and bounces back, giving the light-sensing cells a second chance to catch it.

That's also why a cat's eyes seem to glow when a light shines on them at night. But Britannica points out that cats' eyes don't really glow in the dark: they only shine when light bounces off that mirror layer.

The tapetum lucidum is a reflective layer behind the retina that increases the light available for night vision in many nocturnal vertebrates (Britannica). Light not absorbed on its first pass is reflected back through the photoreceptors, letting cats use ambient light very efficiently; Britannica describes the reflective layer as containing guanine.

Eyeshine is that reflection: cats' eyes do not glow in the dark, they reflect light shone into them.

Warm things give off a kind of light called infrared. Our eyes can't see it.

But special cameras can! They show people and animals glowing in the dark.

There's light all around you that your eyes can't see. NASA explains that infrared light is just beyond the red end of the rainbow. You can't see it, but you can feel some of it as heat.

Warm things, like people and animals, give off infrared light all the time, day and night. NASA says that night-vision goggles and infrared cameras can sense it, so they let us 'see' warm bodies even in total darkness. Even a TV remote uses infrared light, and some phone cameras can see it flash.

NASA describes infrared as the part of the electromagnetic spectrum just beyond visible red: invisible to the human eye but detectable as heat. Objects such as fires are hot enough to emit visible light, while cooler objects such as humans emit only infrared; instruments such as night-vision goggles and infrared cameras let us 'see' that emission from warm bodies.

William Herschel discovered infrared in 1800, measuring a temperature rise just beyond the red end of the spectrum. A TV remote uses near-infrared at about 940 nanometers, which some digital and phone cameras can detect. Earth scientists use the 8 to 15 micrometer band, thermal infrared, to study heat radiating from the planet.