Night Vision: Rods and Rhodopsin
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During the trial at the selection pavilion, Maomao connects the founding myth of the foreign empress dowager, said to have “seen in pitch darkness,” to a precise biological trait: good night vision paired with red-green color confusion. This supposedly divine gift turns out not to be a miracle but a quirk of the retina, plausible in people with red-green color blindness.
The subject in depth
The retina contains two types of photoreceptors. Cones, concentrated at the center, distinguish colors but require abundant light. Rods, far more numerous (roughly one hundred twenty million versus six million cones), are spread across the periphery and are specialized in detecting faint light
What makes rods so sensitive is their pigment: rhodopsin. This molecule is formed from a protein, opsin, bound to a derivative of vitamin A. When a photon strikes rhodopsin, it changes shape and triggers a cascade of nerve signals, even for a tiny amount of light [Wikipedia] . Rhodopsin is then broken down and slowly regenerated, which is why dark adaptation takes time: it takes twenty to thirty minutes in darkness for rods to reach full sensitivity.
Rods do have an important limitation, though: they contain only one type of opsin, insensitive to differences in wavelength between colors. They detect the amount of light, not its hue. Night vision is therefore vision in shades of gray, a universal constant in humans.
Vitamin A deficiency directly disrupts this system. Without a sufficient intake, rhodopsin fails to regenerate properly, resulting in hemeralopia: difficulty seeing in darkness, commonly known as “night blindness.” It’s one of the best-documented consequences of a diet poor in vitamin A.
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