31 A2+ the science of light

CHAPTER 1 See the Light? WHAT IS LIGHT? Lucy stayed up late one night. She slept through her alarm the next morning. Her father came into her bedroom. He turned on the light and shouted, "Get up now, Lucy. It's late!" Lucy answered, "Dad, the light's shining in my eyes. It hurts! Please turn it off." Her father didn't listen to her, "You're going to be late for school! Why are you still in bed?" "Dad," she explained, "I had to work after school. Then I stayed up late studying because I have a test today." Lucy's explanation brought the problem to light. Now her father understood why she was so tired. He felt bad. His daughter was the light of his life, and he didn't like to fight with her. "Oh," he said, "I'm sorry I was so angry. Let's get you to school."

"I know. I can't miss my test!" Lucy said. She looked worried. Her father said, "I know you're working hard. But don't worry, there's a light at the end of the tunnel." Lucy asked, "What do you mean?" "You know, things seem bad right now, but you'll soon be on vacation," her father answered. Lucy said, "Yeah, but right now that feels like light years away!" "It will be here before you know it," her father said, "Now, I give you the green light to sleep as late as you want on Saturday!" Lucy laughed. "Thanks, Dad." The word "light" is used in many expressions. Most of them are about things getting better or looking brighter. This may be because light helps us see, and we couldn't live without it. But what is light exactly?

CHAPTER 2 Night Light IS THERE LIGHT IN SPACE? Have you ever looked at the stars and thought about light? Cosmologists try to answer questions about stars, planets, and other things in space. They think the universe started about 14 billion years ago. They believe that all the matter in the universe was in a small ball. This ball got very hot and expanded really quickly. They call this the Big Bang Theory. Cosmologists believe that a lot of energy came out of the Big Bang. It spread matter out across the universe. This matter became cooler, and after millions of years, stars and planets were created. As the universe expanded and cooled, matter lost energy and some became light. Cosmologists are still learning more about this oldest light in the universe.

Cosmologists know that light comes from stars. The light on the Earth comes from one star, the sun. The Earth turns as it travels around the sun. This movement gives us day and night. When a place on the Earth is facing the sun, it is daytime there. When it is turned away from the sun, it is nighttime. The sun's light also gives us seasons. The Earth takes one year to go round the sun. But the Earth is tilted. So when it travels around the sun, it is summer in the part that is tilted towards the sun and it is winter in the part that is tilted away. Often the moon looks really bright, but it doesn't make its own light. It reflects light from the sun. The sun's light shines on different parts of the moon as the moon travels around the Earth. That is why the moon looks different at different times of the month.

The sun isn't the only star in the sky, but it's the only one we see during the day. We can see about 2,500 stars in the night sky, but there are many more stars in space. Long ago, people looked at groups of stars and saw pictures. They created stories, called myths, about these pictures. For example, people saw two bears in the stars, so they created myths about how the bears got there. Today, we know that stars are made of gases that give off light and heat. Cosmologists can't see all the stars in the universe, but they can see many of them with large telescopes. Now we know there are billions and billions of stars in the universe!

Stars are all different sizes. The sun looks small from the Earth, but it is actually huge it is bigger than one million Earths! Stars are also different colors, too. Hot stars look white or blue. Cooler stars look red or orange. Our sun may feel hot to us, but compared to other stars, it is nice and cool. Cosmologists use telescopes to learn about the stars. The Hubble telescope is special because it is in space. It can see farther than telescopes on Earth. By using the Hubble telescope, cosmologists learned about a star cluster called NGC 6397. The stars in this cluster are so close together that two of them sometimes hit each other. When two stars hit each other, they become one star that is very bright.