homehome Home chatchat Notifications


Infrared light can be detected by the human eye after all

The human retina can only detect incident light that falls in waves 400 to 720 nanometers long, so we can’t see microwave or ultraviolet wavelengths. This also applies to infrared lights which has wavelengths longer than visible and shorter than microwaves, thus being invisible to the human eye. Apparently, this isn’t entirely true. In some special conditions, the […]

Dragos Mitrica
December 9, 2014 @ 1:52 pm

share Share

The human retina can only detect incident light that falls in waves 400 to 720 nanometers long, so we can’t see microwave or ultraviolet wavelengths. This also applies to infrared lights which has wavelengths longer than visible and shorter than microwaves, thus being invisible to the human eye. Apparently, this isn’t entirely true. In some special conditions, the human eye can indeed detect infrared light according to scientists at Washington University School of Medicine in St. Louis.

Not that invisible after all

The human eye can detect only those light frequencies that fall in the so-called visible spectrum. Anything outside of it, like the ultraviolet or infrared range, can't be detected, hence seen. In some special conditions, however, scientists have demonstrated that its possible to notice infrared light.

The human eye can detect only those light frequencies that fall in the so-called visible spectrum. Anything outside of it, like the ultraviolet or infrared range, can’t be detected, hence seen. In some special conditions, however, scientists have demonstrated that its possible to notice infrared light. Image: Sara Dickherber

It all started when researchers started seeing brief flashes of green light from time to time while working with an infrared laser. These light waves are thought to be invisible to the naked eye, so everybody was very intrigued by this episode. If it didn’t happen to more than one person in the lab, most likely the whole event would’ve been put on fatigue or a sudden lapse of insanity.

Being scientists, the US researchers sought to investigate the matter. They soon came across scientific literature that reported how some people were able to detect infrared light when exposed to infrared lasers. They soon geared some experiments where various lasers where used, operating under different parameters. They found the shorter the pulse, the likelier was for a person to notice the infrared light.

“We experimented with laser pulses of different durations that delivered the same total number of photons, and we found that the shorter the pulse, the more likely it was a person could see it,” Vinberg explained. “Although the length of time between pulses was so short that it couldn’t be noticed by the naked eye, the existence of those pulses was very important in allowing people to see this invisible light.”

Past the visual spectrum

A thermal image visually represents the difference in temperature across various surfaces on a body. Image: Wikimedia

A thermal image visually represents the difference in temperature across various surfaces on a body. Image: Wikimedia

The primary source of infrared radiation is heat or thermal radiation. This is the radiation produced by the motion of atoms and molecules in an object. The higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce. Humans, at normal body temperature, radiate most strongly in the infrared, at a wavelength of about 10 microns (a micron is one millionth of a meter). Using a thermographic camera you can see infrared radiation, but that’s a bit of an overstatement. Simply put, information about the radiation is being represented visually for humans, who can’t see in infrared. Some animals can “see” in the infrared. For example, snakes in the pit viper family (e.g. rattlesnakes) have sensory “pits,” which are used to detect infrared light. This allows the snake to find warm-blooded animals (even in dark burrows), by detecting the infrared heat that they radiate. S

At the back of the eyeball is a layer of light-sensitive cells, called the retina. Photons (packets of energy) in lightwaves stimulate the cells of the retina and send messages to the brain, enabling us to see. The retina’s rod-shaped cells work best in dim light, and its cone-shaped cells, which detect colour, in bright light. The photons aren’t absorbed directly, though. In an intermediate step, photons absorbed by the retina stimulate the creation  a molecule called a photopigment, which begins the process of converting light into vision. Typically, each photopigment corresponds to one photon.

If we pack a lot of photons in a short pulse, like with an infrared laser, then its possible for one two photons to be absorbed at one time by a single photopigment, and the combined energy of the two light particles is enough to activate the pigment and allow the eye to see what normally is invisible.

“The visible spectrum includes waves of light that are 400-720 nanometers long,” explained Kefalov, an associate professor of ophthalmology and visual sciences. “But if a pigment molecule in the retina is hit in rapid succession by a pair of photons that are 1,000 nanometers long, those light particles will deliver the same amount of energy as a single hit from a 500-nanometer photon, which is well within the . That’s how we are able to see it.”

There’s some practical applications to the findings (PNAS), too. Doctors might want to shine infrared pulses into a patient’s retina to see if it responds correctly to the stimulus.

 

share Share

Huge Study Links Ayahuasca to Mental Health Benefits—But It’s Not for Everyone

Naturalistic use of this Amazonian brew shows potential mental health benefits, but with risks.

Women Didn’t Live Longer Than Men in Medieval Times. Here's Why

Bones tell the story of gender and survival in Medieval London.

This hidden mineral is crumbling thousands of home foundations across New England. “It’s like your house was diagnosed with cancer”

Pyrrhotite causes cracks in concrete. But research on how widespread the issue might be has only scratched the surface.

Roman-Era Britons Had Scandinavian DNA Long Before Viking Raids

Centuries before the Vikings, Scandinavian roots intertwined with Britain's ancient history.

Loneliness makes you more prone to disease. Interacting with friends and family can help

Social isolation and loneliness are more than personal struggles—they're global public health crises.

Why Winter Smells So Fresh: The Science Behind the Seasonal Aroma

Ever noticed how winter air smells so uniquely crisp and fresh? It’s not just your imagination.

Scientists Achieve Quantum Teleportation Using Existing Internet Cables

Researchers demonstrate quantum teleportation over internet traffic, paving the way for secure applications.

9 in 10 new cars sold in Norway in 2024 were electric

Norway’s bold policies and long-term vision have turned it into a global leader in electric vehicle adoption.

This Radar System Can Detect Hidden Moisture in Your Walls

Mold is one of the most significant challenges for homeowners, and once it takes hold, it can be incredibly difficult to eliminate. Preventing mold is the best approach, and the cornerstone of mold prevention is managing humidity. Now, researchers from Oak Ridge National Laboratory (ORNL) have developed a method using microwave radar to monitor the […]

The surprising link between your pupils and how your brain stores memories at night

In the stillness of sleep, tiny pupil shifts in mice uncover a remarkable secret: the brain’s delicate act of preserving memories while forging new ones.