What Eyes Can See How Do You See What You See?

Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. Image Credit: Shutterstock Animals eyes can look very different on the outside, and their internal structures are different too. Different eyes allow different animals to see different types and amounts of light. What Eyes Can See How Do You See What You See? The eye is a detector of visible light. A detector of visible light is something that can tell when visible light is present it doesn t make its own light. Solar panels are also detectors of light: they absorb light from other sources but don t make their own. Because your eyes detect visible light, you don t see very well when there isn t much light around, like at night or in a dark room. Something similar happens when light is blocked: light can only move in a straight line, which means it can t curve around corners or things that get in its path. That s why you see a hand-shaped shadow on the wall when you hold your hand up to a video projector: light travels in a straight line from the projector bulb and continues to the nearest wall, except where your hand blocks it, making a dark spot shaped like your hand. When light hits a material, three different things can happen: the material can absorb the light (take it in), transmit the light (let it pass through), or reflect the light (cause it to bounce off). Depending on the material, all three things may happen at once. These three possibilities determine the way you see: light moves from a source to your eye in a series of straight lines, passing through some materials and bouncing off others, before the light is finally absorbed by the back of your eyeball. There, tiny organs called rods and cones absorb energy from the light. This energy is converted into a chemical signal with information about the color and brightness of the light. That chemical signal sends an electrical message to the brain about the light it s receiving, and the brain interprets the message as vision. What Eyes Can See 1

This explains how you are able to see, but it doesn t explain why you see certain objects as certain colors. For instance, why are bananas yellow? Bananas look yellow because they reflect yellow wavelengths of light, bouncing the energy from these wavelengths off rather than taking it in. When you look at a banana, reflected yellow wavelengths hit your eye, where their energy is absorbed. The energy you take in from these wavelengths sends a signal to your brain, which you interpret as the characteristic color of bananas: yellow. When light hits a material, the light can be absorbed into the material, transmitted through the material, or reflected away from the material. What about when you look at something black, like a crow, or something white, like an egg? People usually think of black and white as colors, just like yellow. To your eyes, however, black and white are different from the colors of the rainbow. Black is actually the absence of color. Black objects absorb all wavelengths of visible light. Taking in energy from so many different wavelengths can make black clothes feel hot and uncomfortable on sunny days. White, on the other hand, is actually the combination of all colors. White objects reflect all wavelengths of visible light. White clothes are great for staying cool on a sunny day because they don t take in any energy from visible wavelengths. If black is the absence of color, what about things that appear to have no color, like a clear piece of glass? Clear glass seems to have no color of its own, but it isn t black, either. When you look through a piece of clear glass, you see the colors of objects on the other side. This is because clear glass transmits all wavelengths of visible light. When these wavelengths hit a pane of glass, they pass right through, carrying their energy across to the other side. Through a clear glass window, you see a world full of colorful objects, each of which is reflecting certain wavelengths of light through the glass to your eye. The color of an object is determined by the way it absorbs and reflects light. White objects, like this egg, reflect all visible wavelengths of light. Black objects, like this rock, do not reflect any visible wavelengths of light. To your eyes, white is the combination of all colors, and black is the absence of color. Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. 2 What Eyes Can See

Of course, humans are not the only organisms with eyes...but that doesn t mean that all organisms see the world the same way humans do. Select one of the chapters that follow to learn about how another organism sees. Clear glass transmits all colors of light, so your eyes see the colors of the objects behind the glass instead of the glass itself. Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. What Eyes Can See 3

Do Dogs See Red? If you throw a red ball into a big green lawn, your dog might take a little longer to find it. That s because, to a dog, red and green look exactly the same and they don t look the way red or green look to us. Dogs see differently from humans because their eyes are different. Both dogs and humans have tiny organs inside the eye called cones, but our cones are different. Humans have three kinds of cones: one kind absorbs mostly red wavelengths of light, one absorbs mostly green wavelengths, and the last absorbs mostly blue wavelengths. That combination of cones allows us to see all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Dogs, on the other hand, only have two kinds of cones: one kind that absorbs mostly yellow light and one kind that absorbs mostly blue light. That means dogs only see different colors and combinations of yellow and blue. Because they don t have red-sensitive or green-sensitive cones, dogs don t see red or green and instead see shades of gray. Scientists have found evidence that dogs use brightness cues to tell red and green apart, since the gray that dogs see in place of red tends to be brighter than the gray they see in place of green. To dogs, red and green look exactly the same. A dog may take longer to find a red ball in green grass than it would to find a ball of another color. Human eyes have cones that absorb red light, blue light, and green light. We see the whole spectrum of visible light. Dogs eyes have cones that absorb yellow light and blue light. They mostly see yellow and blue. Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. Image Credit: (t) Shutterstock 4 What Eyes Can See

The Mantis Shrimp Has Cones for Many Colors Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. Image Credit: (t) Getty Images When you open your eyes, you probably see all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). The colors you see are thanks to the cones in your eyes the tiny organs that allow you to see color. Unless you re color-blind, your eyes have three kinds of cones: cones that absorb mostly red light, cones that absorb mostly green light, and cones that absorb mostly blue light. Compared to many animals, humans see a wide variety of colors. Still, there s one kind of animal that has much better color vision than humans (or any other animal we know of): the mantis shrimp. A mantis shrimp has anywhere from 12 to 16 different types of cones in its eyes, and it sees many, many more colors than you do! Mantis shrimp can see the visible spectrum (all the colors humans see) and also ultraviolet light. That means that they can see light that has shorter wavelengths than the parts of the spectrum humans can see. Scientists have some evidence that the mantis shrimp s eyes evolved to help it catch food: the mantis shrimp is a fierce hunter with lightning-quick reflexes. However, nobody knows exactly how seeing ultraviolet colors actually helps the mantis shrimp. The eyes of a mantis shrimp have many more types of cones than other animals eyes do. That s why a mantis shrimp has excellent color vision. Human eyes have cones that absorb visible light. Mantis shrimp eyes have many more types of cones than humans do, allowing them to see all visible light as well as some ultraviolet light. What Eyes Can See 5

What a Bee Sees When you look at a flower, you see different colors than a bee does. Why? Inside your eyes are tiny organs called cones. Cones are what allow you to see all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Unless you re color-blind, your eyes contain three kinds of cones: cones that absorb mostly red light, cones that absorb mostly blue light, and cones that absorb mostly green light. Bees also have three types of cones inside their eyes, but their cones are different from the ones inside a human s eyes. Bees have cones that absorb mostly blue light, cones that absorb mostly green light, and cones that absorb mostly ultraviolet light. (Cones that absorb ultraviolet light respond to colors with shorter wavelengths than violet in the spectrum of light. Humans can t see ultraviolet light.) What bees can see overlaps with the spectrum that humans can see, but it s closer to the violet end bees don t see red, but they do see purples that we don t. Some flowers that look plain to humans actually have patterns in colors that are invisible to humans. We can t see these patterns, but bees can. Ultraviolet patterns are often at the center of the flowers, and scientists have some evidence that the patterns help bees land close to the nectar they re looking for. Bees have cones that absorb mostly red light, cones that absorb mostly blue light, and cones that absorb mostly green light. The cones in human eyes absorb visible wavelengths of light. The cones in bees eyes do not absorb red light, but they do absorb ultraviolet light that humans don t see. Light Waves 2015 The Regents of the University of California Permission granted to purchaser to photocopy for classroom use. Image Credit: (t) Shutterstock 6 What Eyes Can See