When night falls, three changes occur to the eyes. One, humans’ color vision worsens; two, it becomes difficult to focus close-up; and three, peripheral vision improves. When human’s ancestors moved around in the dark, they didn’t necessarily need to see details, but they did need to know if someone (or something) was sneaking up on them.
How Human Eyes See in the Dark
When it becomes dark, the pupil begins to dilate to allow more light into the eye. It takes approximately fifteen minutes for the pupil to dilate completely. In the back of the eye, the retina takes on a purplish tinge as the color-sensitive cone cells of the daytime sight start to recede. The light-sensitive rod cells around the edges of the retina become active. Forty-five minutes pass before the visual purple or Rhodopsin, the pigment of the rods, saturates the rod cells, resulting in the highest-quality night vision.
Night Vision in Animals
Since the rod cells of the eye’s light-catching retina are sensitive to even dim light, the eyes of many nocturnal animals are packed with rod cells. Most crepuscular (active at dawn and dusk) and nocturnal animals can see well enough at night to find and catch food, flee from predators and navigate around objects.
Many nocturnal animals have only a few color-sensing cone cells because there is usually not enough light present for color to be detected at night. The lighter colors by day, such as reds and yellows, appear as darker grays at night, whereas blue, which is not a bright during the day, appears as a brighter shade of gray after dark. Human eyes are especially sensitive to blue at night.
Eye Shine
Animals whose eyes seem to glow in the dark have a reflective layer of tissue called the tapetum lucidum at the back of the eye, behind the retina. Light bounces off the tapetum lucidum to increase the eye’s efficiency in low light levels. A bright light, like a flashlight, reflects off the animal’s eye, the creature’s eyes seem to glow. Each kind of animal has eye shine of a particular color.
Test People’s Night Vision
Create a pirate patch and place it over one eye for at least forty-five minutes while remaining outdoors in bright light. When the time is up, go into a darkened room, such as a basement. Remove the patch. Alternate opening and closing one eye at a time. The eye covered by the patch still has its night vision. The dark room will appear bright compared to the eye that has moved from bright light into the dark.
Disappearing Faces
Partners stand arms’ length from one another and stare at each another’s nose. A partner’s face should appear to disappear, although the person’s hat/hair and jacket will remain. The cone cells in the center of the eye pick up color. At night, the human eye can’t see color so by looking straight ahead, the cells can’t pick up the color of the person’s face and is seems to vanish.
Seeing Color at Night
At night, take a walk or play outside in a spot where there is little light. After twenty-to-forty minutes, hand out index cards and crayons with the wrapper removed. Participants write down their name and the color they believe the crayon to be on the index card. Collect the crayons and cards. Players find out if they are correct when they go inside where there’s light.
With a few activities, children can learn about how humans and animals can see at night. Children can explore their night vision, the improvements in their peripheral vision and the decrease in their color vision.
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