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A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than a cone cell. Since rod cells require less light to function than cone cells, they are therefore the primary source of visual information at night (scotopic vision). Cone cells, on the other hand, require tens to hundreds of photons to become activated . . . peripheral vision (is) very sensitive to movement, and is responsible for the phenomenon of an individual seeing something vague occur out of the corner of his or her eye.
Experiments by George Wald and others showed that rods are most sensitive to wavelengths of light around 498 nm (green-blue), and insensitive to wavelengths longer than about 640 nm (red).
Rod cells also respond more slowly to light than do cone cells, so stimuli received by rod cells are added over about 100 milliseconds. While this makes rods more sensitive to smaller amounts of light, it also means that their ability to sense temporal changes, such as quickly changing images, is less accurate than that of cones (Kandel et al. 2000). However, if multiple flashes of sub-threshold light occur during the 100 millisecond period, the energy of the flashes of light would aggregate to produce a light that will reach threshold and send a signal to the brain.
Flicker fusion thresholds decline towards the periphery, but do that at a lower rate than other visual functions; so the periphery has a relative advantage at noticing flicker. Peripheral vision is also relatively good at detecting motion.
In some special conditions, the human eye can indeed detect infrared light according to scientists at Washington University School of Medicine in St. Louis. “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.”
“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 visible spectrum. That’s how we are able to see it.”