Humans Seeing Infrared Light...Kind Of
Researchers have shown definitively that not only can humans see infrared light but they determined how we can see it.
Does this come as a surprise to you? It should.
Read any textbook on this and they clearly state that for humans, this does not happen. Like much of science though, there’s interesting exceptions.
The entire electromagnetic spectrum of light is huge, spanning from gamma rays on one end to radio waves at the other with X-rays ultraviolet etc in between including of course that tiny slice called Visible Light, The light we can see. The “Visible Spectrum” is a subjective term of course. Visiting aliens looking at how we split up the spectrum would wonder why the hell we split it up that way. Perhaps a better name would be the Human Spectrum
This tiny human spectrum encompasses a very specific range of wavelengths from about 380 nm to 780 nm. There are many reasons why this is so but two big ones include the fact that the sun produces a lot of light in that range and our atmosphere doesn’t filter out any of it.
Physiologically we see these frequencies because the photoreceptors in our retinas are sensitive to them (duh). When photons of light hit the photoreceptors it creates an electrochemical signal which is the first step in a fascinating process called the visual transduction cascade which ultimately results in us seeing color.
Seeing other frequencies would be cool of course but they would generally not be very helpful to us or wouldn’t work well, Infrared for example would be a problem because the infrared or heat generated in our heads would constantly be triggering our vision. This would not be a problem for cold blooded animals which is why some of them have evolved to see it.
Still though it is possible for our vision to be triggered by Infrared light. Experiments in the past using lasers clearly showed this is possible but it was never studied systematically and no knew what the specific mechanism was.
This is exactly what Washington University researchers recently accomplished.
They started by going over the designs of these past experiments and any literature mentioning the visible infrared light. They then used retina cells from mice and humans to try and replicate this effect for themselves.This was done using a powerful and pure infrared laser which they fired at the retina cells using various types of pulses until they reproduced the effect. It turns out that extremely short pulses of light are required to reliably allow the cells to be triggered by the laser.
Once they verified that this effect was real they then needed to determine what the mechanism was. Often, that’s the most fascinating part of a new discovery. To fully appreciate what they found, keep in mind that normally, a photopigment molecule in a retina is activated by one photon of light. In their experiments though, if a pulse of photons from the laser was short and dense enough, two infrared photons were absorbed by one molecule at the same time. So since two photons of 1000 nm infrared light have the same energy as one photon of 500 nm visible light, they could trigger the photoreceptor, resulting in the perception of a color.
Unfortunately, you wouldn’t see a new never before seen color, it’s plain old green. This is because the infrared light is triggering a visible light photoreceptor. This newly-discovered process essentially then moves invisible light into our visible spectrum so we can see it
To actually see the infrared as it really is would be impossible for humans. To pull that off we’d need different genes encoding for a new photoreceptor cells that baseline humans simply do not have.
I hate baseline
Image Credit: Physorg.com and http://scienceblogs.com/startswithabang/2011/09/14/the-color-of-space/