Humans can't see infrared. That's why we fear animals like snakes, bed bugs, and the Predator. No longer must we live with this fear! Scientists have shown that, under certain circumstances, our retinas actually can detect infrared light.
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How Animals Sense Infrared
It's worth noting that, though there are numerous animals that sense infrared light, relatively few of them sense it with their eyes. Snakes evolved infrared "vision" twice. The older boids, a class that includes boas and pythons, have pits lined with heat sensors along their upper and lower jaws. The crotalines, pit vipers, have a sensor-lined membrane stretched above a pit between their eyes and nose.
It looks as though the information from these pits combines with vision from the retinas in the vision center of the brain, so they probably do see it. Vampire bats also sense the infrared radiation given off by their prey with pits around their nose.
Bed bugs carry their infrared sensor on their antenna. And a certain type of beetle, attracted to fires because it lays its eggs in burnt wood, has pits similar to the boa constrictor. Although all these animals do have eyes, and some have what we would consider "heat vision," none of them use their optic retina to obtain this heat vision.
What is That Light?
Scientists at the Washington University School of Medicine in St. Louis were happy to be working with a powerful new infrared laser. One of the important things they needed to do while working with it was checking to see if the laser was off or on. Eyeballing the machine wasn't going to cut it, as the infrared isn't visible to human eyes.
Which made it odd that people around the lab were seeing flashes of green light when the laser was on. The flashes disappeared whenever the laser turned off again. How could people, without even trying, see infrared light with their retinas when not even animals see infrared light with their retinas? And why was the light green?
The Experiments in Infrared
An international team of scientists decided to experiment with humans and lasers. They used different infrared lasers to flash light at people. The flashes were carefully calculated so that each would give people the same amount of photons streaming towards their eyes, but those photons would come in different intervals of time. A short amount of time meant the infrared photons came in a flood. A long amount of time allowed photons to trickle through the subjects' retinas.
Inside the human eyes are photopigments - pigments that change structure when they get hit by a photon. The part of the photopigment that changes its structure is the chromophore. It is shackled to what's called an opsin. Give a chromophore just the right amount of energy, and it changes its structure, cutting the opsin loose and starting the process that ends with what we call "seeing." The only photons with the right amount of energy to change a human chromophore are in the 390-720 nanometer wavelength range. Infrared, in the 1000 nanometer wavelength range, is too big and too low-energy to knock a chromophore into changing its shape.
But if huge amounts of infrared photons flooded the eye over a short period of time, two infrared photons could hit the chromophore at once. Their combined energy is enough to cause it to change its structure and allow people to see what they otherwise wouldn't. Two 1000 nanometer photons add up, energetically speaking, to one photon of around 500 nanometers - which is in the green range of the visual spectrum. So infrared light, if concentrated enough, would leave us seeing green.