The human eye is a more complex and mysterious thing than we thought. Recently, a group of scientists were puzzled by flashes of green light they saw from an infrared laser, whose light should have been far outside the visible spectrum. Like scientists do, they investigated. Human eyes do indeed perceive infrared light, they found, but not they same way they perceive ordinary colours. It's weirder than that.
Their study, published this week in the journal PNAS, suggests it has to do with photons of infrared light doubling up. Infrared light has less energy than red, blue, green, or any colour we consider in the visible spectrum, so it can't excite the photoreceptors in our eyes. But if two photons of infrared light hit the same receptor one right after another, their energies add up to one photon of visible light. Hence, the "green" from an infrared laser.
Fancy two-photon microscopes actually work on the same basic idea. Two photons of infrared light together excite a fluorescent molecule in a sample, causing it to glow. Infrared light has the advantage of penetrating deeper into a solid sample, so the resulting image is much clearer than you'd get with an ordinary microscope. We figured out how to make two photons work together in lab equipment before we figured out it may literally be happening in our eyes.
But right, this isn't exactly night vision — all it really works on are the focused beams of a laser. Eye doctors are interested in it as a new way to probe photoreceptors in the eye. And biohackers who are trying to modify their vision to see infrared, well, they have some more information to work with.
Perhaps most fascinating of all, though, is that this demonstrates how human vision is our hyperconstructed interpretation of reality. It's easy to believe that our eyes perceive the world as it really is, but nope. Just look at how optical illusions fool us. The same colour appears different on different backgrounds, lines appear to move on their own, etc etc. Our visual system is constantly trying to find patterns in the photons streaming into our eyes — we see its interpretation of the world, not the world as it really is. [PNAS via WashU]