Night Sight At night, eyes need extra help to catch light. Special cells in the retina come to the rescue. Tey are called rods. Rods tell the brain that light is hitting the retina. Tese cells help the eye see in shades of gray. Each human eye has about 125 million rods! Even with all that help, it’s still hard for
humans to see well at night. It’s easier for nocturnal animals. Tese animals are active at night. Many have extra large eyes and pupils. Te tarsier is a good example. It’s a small,
nocturnal primate. Its eyes are the largest of any mammal, relative to body size. If a human’s eyes were proportionally as big
as those of the tarsier, they would be the size of grapefruits. Larger eyes catch more light. Te tarsier’s night vision is excellent. Others animals, like cats, have eyes that
seem to glow. Tey have an extra part in their eyes. It acts like a mirror. Tese eyes catch light twice: first as it enters the eye, and then when it reflects the light.
The Colors of Light In dim light, the world looks gray. It takes more light to see color. In fact, what looks like white light is really made up of many colors. Remember the light waves? Each
wavelength has a different color. Te picture to the right shows what happens when white light enters a prism. Te clear glass bends and separates the 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. Many animals can’t see the same ones humans 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. Te eye sees yellow.
Color Vision It takes more than just light to see color, though. It also takes cones. Each human retina has about 7 million cones. Tese cones are most sensitive to red, green, and blue wavelengths. Light waves hit the cones and cause a reaction. Te cones then send color messages to the brain. Te brain can mix the colors together. Tat’s why the eye can see many colors. Some animals have other kinds of cones.
Scientists think these animals see in ways humans can’t. A bee is a good example. It may see colors and patterns on flowers made by ultraviolet light—light waves humans can’t see. Te light may help the bee find a spot to land. Other animals, like the giant squid, don’t
have any cones. Tese animals only see in black and white. Yet the squid has extra rods. Tat helps its eyes sense as much light as possible in the dark ocean. Rods and cones work together to help eyes
Similar to a tarsier, this slender loris has extra big eyes to help it see in the dark.
20 NATIONAL GEOGRAPHIC EXPLORER
see. If a person moves from a well-lit room to a darkened room, it may take as much as 10 minutes for the cones to adjust and take over. Until that happens, it might be hard to see.
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