Night Sight Neither humans nor any other animal can see in absolute darkness. So in the dim light of night, eyes need to capture as much light as possible and make the most of it. Light-sensing rod-shaped cells in the retina do most of the work. Rods sense light, but not color, and they detect motion. Each human eye has about 125 million rods. Many nocturnal animals, or those active
at night, have extra large eyes like the tarsier. Tis small, nocturnal primate has the largest eyes of any mammal relative to body size. If a human’s eyes were proportionally as big as those of the tarsier, our eyes would be the size of grapefruits. Large eyes collect more light, giving the tarsier excellent night vision. Others animals, like cats and deer, have eyes
that seem to glow. Teir eyes have an extra layer behind the retina that acts like a mirror. It gives the retina a second chance to capture light and send a nerve impulse. We see this reflecting layer through the animal’s pupil.
The Colors of Light Light and color go hand in hand—you can’t have one without the other. White light itself is really all of the colors of the spectrum blended together. It’s easy to show with a prism that refracts white light into a rainbow of colors. Each light wave has a specific color based on
how long the wave is from peak to peak. Some wavelengths humans can see, while others they can’t. And many animals can’t see the same ones we do. Te other two ways light behaves—reflecting
and absorbing—cause an object to appear a certain color. A bright yellow T-shirt absorbs all light waves except the yellow ones. Tose it reflects. If you’re looking at it, the yellow light waves reflect into your eyes.
Color Vision It takes more than light to see color. Te retina also contains color-sensing cone-shaped cells. In humans, cones are most sensitive to red, green, and blue wavelengths. Each human retina has about 7 million cones. Light waves hit cones and cause a reaction.
Te cones then send color messages to the brain. Te brain mixes the relative amounts of red, green, and blue messages together. So the eye can see more than those three colors. Some animals have other kinds of cones, so
these animals see in ways humans can’t. Bees, for example, see colors and patterns on flowers made by ultraviolet light—light waves humans can’t see. Te colors and patterns increase the chances that the bee lands where the nectar is. Other animals, like the giant squid, don’t
have any cones at all. Yet the squid has extra rods. Tis helps its eyes sense as much light as possible in the dark ocean. Rods and cones work together in the eye to
Similar to the tarsier, this slender loris has large eyes that allow it to see well at night.
14 NATIONAL GEOGRAPHIC EXTREME EXPLORER
give the brain as much information as possible. If a person moves from a well-lit room to a dark room, it may take as long as 10 minutes for the cones to adjust to the dark and 30 minutes for the rods to adjust. Until that happens, it is hard to see. Te reverse happens when moving from a dark space to a light space.
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