One day in the early 1660s, Isaac

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1 Clark Discovers One day in the early 1660s, Isaac Newton spent a rare few hours outside, at the annual market near his college. He bought a toy a prism, a piece of glass cut at precise angles. Though he had next to no money, the prism was so cool he bought another one. He was not yet Isaac Newton the greatest scientist ever. He was merely an unpopular, solitary, brilliant college student. Then the plague hit England hard, and the only way to avoid catching it was to avoid other people not a problem for the unsociable Newton. In 1666, at age 24, he was forced to leave college and retreat to his remote childhood home with his prisms and other science toys. While other students might have goofed off, Newton sat still...and thought. Which of the many puzzles in nature could he solve while he was waiting out the plague? All was quiet except for the bleating of sheep. He lived in a time and place with no distractions no TV, computers, cell phones, video games, or malls. The sparkling prisms caught his eye. What if he could understand the nature of color something more accurate than what he was being taught in college? i8 ask

2 Not Just Black and White Ever since the ancient Greek Aristotle said so, scholars assumed that white light is one simple, uniform, solid thing. Colors, therefore, must be white light mixed with something maybe "blackness." Even in Newton's day, many scientists, such as Newton's future rival ) Robert Hooke, taught that colors ' are a mixture of light and dark. Hooke had invented a color scale ranging from bright red, which he claimed was pure white light mixed with the tiniest amount of darkness, to soft blue and then black, which he thought was Theh why don't we see colors the dark?

3 darkness completely blocking out the ) sunlight fell on them. They believed that white light. I the prism itself was somehow coloring Newton didn't think Aristotle or ' the light. Hooke could be right about color. After In his lonely study upstairs at his all, a white page with black writing does mother s farmhouse, Newton set out not appear in color when viewed from his prism in front of a white screen in a a distance. Rather, the black and white darkened room and made a small hole blend into gray. So he set out to prove in the window-shade to let in a skinny the experts wrong one of his very beam of sunlight. As the light passed favorite activities. through the prism, he observed that The prism was the perfect tool for the beam spread out into an oblong of his experiment. Others, like Hooke, were colored bands of light, which he called a using prisms too, admiring the spectrum. The white light had split into colors they projected when different colors. How? 20 a s k

4 Why Do Things Look Different Colors? NVhen light hits an object, some ofthat light is absorbed by the object, and some bounces off (and hits our eye, letting us see things). But different things absorb and reflect different wavelengths (colors) of light. The color that you see is made by the combined colors of the reflected light. An object that looks red, for example, absorbs all the wavelengths of light except the red wavelength. That bounces off to your eye, and you see red. When you dye or paint something to change its color, you are adding molecules that reflect the desired wavelengths of light. Making Rainbows Newton kept thinking. He imagined each color as a different kind of light ray maybe with different sizes or energies. Newton noticed that whether he made rainbows with a prism or with a pan of water, the colors always spread out in the same order. Violet and blue beams bent the most, while red bent the least. Maybe transparent substances such as glass and water were bending the path of each type of ray by a different amount as it passed through. What if all the colors are present in white light, and the prism simply fans them out according to how much the glass bends them? To test this idea, he added a lens behind the prism to focus the rainbow together again into a single point on the wall and got white. He had proved that white light contains all the other colors. This was huge. Newton divided up the spectrum into seven bands, giving us the rainbow we know today: red, orange, yellow, green, blue, indigo, and violet though he could have picked five bands, or 50. He also realized that the colors we see are the result of objects reflecting just some of the colors that make up white light. Juçt [^[ïiember l?oy G. &v: Red, Orange, \ Yellow, Green, Blue, /ndigo, and Violet. ) ^ ask

5 The Eyes Have It In the 1960s, scientists discovered three types of color receptors, called cones, in the retina at the back of the human eye. The cones are slightly different shapes and are sensitive to the red, blue, and green wavelengths of light, which are light's primary colors. Different combinations of these hues let us see any color. Today it is believed that the eye can perceive over 2.8 million different hues more colors than even Newton could imagine. Green Li^ht, Green Light In his excitement, Newton forgot to eat, forgot to sleep. So far, so good. Now, to prove that the prism did not color the light, Newton did an "Experimentum Crucis." No, this wasn't a spell stolen from Harry Potter, but what scientists call a crucial experiment. Newton cut a small slit in the screen behind his prism. When the prism made a rainbow, he allowed only the green light to pass through the slit. Then he grabbed his second prism and placed it in the green light. If the prism was coloring the light, the green would come out a different color. But the green light remained green. The second prism had no effect. Newton rarely smiled, but he might have then. He couldn't brag to anyone yet his rundown farmhouse was about a mile from the nearest road. But he had just established that colors are governed by scientific principles, and he suspected the rest of the natural world is too. And he had become the first person to really under- ^ ' stand the rainbow. Time to go think some more under his apple tree a s k

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