The “visible” portion of the electromagnetic spectrum is so named because that is the range detected by normal human vision, ranging from 400-720 nanometers. Some previous studies have indicated that under certain conditions, humans can perceive near infrared wavelengths as visible light, and this study has found a mechanism that explains the phenomenon. The research was led by Krzysztof Palczewski of Case Western Reserve University in Cleveland, Ohio, and the results were published in the Proceedings of the National Academy of Sciences.
“A few previous reports and our expanded psychophysical studies here reveal that humans can detect IR at wavelengths longer than 1,000 nm and perceive it as visible light, a finding that has not received a satisfactory physical explanation,” the authors said in the paper.
For this study, 30 volunteers had beams of near-infrared light pulsed into their eyes. All participants were able to see the light as color. The wavelengths that were longer were perceived as being reddish in color, while shorter wavelengths were seen as green.
Normal vision is the result of light in the visible wavelength spectrum entering the eye, getting focused by the lens onto the rod and cone cells of the retina, and having those impulses sent to the back to the brain for processing. While normal human photoreceptor sensitivity drops off past 720 nanometers, Palczewski’s team was surprised to learn that the sensitivity of the cells jumped back up with wavelengths longer than 900 nanometers, depending on the power of the laser.
Experiments were then performed on chromophore within mammalian eyes. They found that while chromophores typically need to absorb visible light in order to activate the pigment protein rhodopsin, the same effect can be achieved with near-infrared wavelengths, provided that two photons work together at the same time.
“These observations are consistent with our quantum mechanical model for the energetics of two-photon activation of rhodopsin,” the paper continued. “Thus, humans can perceive IR light via two-photon isomerization of visual pigment chromophores.”
In addition being a pretty fun trick, there are some useful clinical applications that could arise from understanding this phenomenon. Popular Mechanics reports that beams of near-infrared light could be used to detect the earliest stages eye damage, when there is still time to intervene and protect the patient's vision. Visible light could not work in this application, because it would be far too damaging.
[Hat tip: Popular Mechanics]
[Image credit: Sarah Scicluna via Flickr, CC BY-ND 2.0]
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