If you’ve driven by a police car parked on the side of the road at night and noticed a red light coming from inside, it’s not because the officer is getting abducted by aliens or some other oddity. The real reason, while much more straightforward, is nonetheless quite interesting and helps minimize the danger to the officer.
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Humans are best suited for going about their business in the daytime because we are diurnal (or active during the daytime) by nature. As such, our eyes have adapted to work better in the daylight than at night. The eyes of both diurnal and nocturnal creatures have pupils that dilate/constrict to control light, lenses that focus images, and nerves that transmit that image back to the brain. They also contain light-sensing cells called cones and rods. Humans have more cones than rods because their primary use is seeing color, while nocturnal animals have more rods than cones because they’re better at picking up vague images in the inky blackness of night.
Natural night vision (as opposed to artificial night vision created by binoculars) is created not only by the dilation of the pupils but also by a protein called rhodopsin, which can’t be made during daylight hours because it is too bright. Only once the eye has detected low levels of light (usually after 20 minutes, give or take) will it begin to produce rhodopsin. Unfortunately, it takes another 20 – 40 minutes before the rhodopsin kicks in to create night vision. What’s more, a quick blast from bright white light — say from your standard flashlight, headlamp, or headlights — will destroy that night vision, and it’ll take another hour or so before it returns.
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