The human eye can detect a single photon in total darkness
Under ideal conditions — fully dark-adapted, with the eye at rest — the human visual system can detect as few as 1–5 photons. A 2016 study published in Nature Communications confirmed this by having subjects detect single photons fired at their eyes and correctly identify their presence above chance levels. The eye's rod cells (responsible for night vision) are sensitive enough to register a single quantum of light. The practical limit is that the brain filters out most of this signal as 'noise', but the physical capability is there.
The photon is the smallest possible unit of light — a single quantum. Discovering that the eye is sensitive to an individual photon makes human vision feel simultaneously more miraculous and more fragile than we usually consider it.
“The human eye can detect a single photon in the dark — confirmed by a 2016 study. One quantum of light. That's the smallest possible unit of illumination, and your eye can register it. 👁️ #OddlyHuman”