20. Waves

                              Waves
How does light get from the sun to the earth? How does music get from the stage to the audience? They move the same way-----in waves!
Light and sound are forms of energy. All waves carry energy, but they may carry it differently. Light and sound travel through different kinds of matter. For example, light waves cannot move through walls, but sound waves can. That is why you can hear people talking in another room even though you cannot see them. The energy of some waves is destructive. An earthquake produces seismic waves.
   Catch a wave. Ask a friend to stand a few feet away from you. Stretch a spring between you. Shake the spring to transfer energy to it. What happens? The spring bounces up and down in waves. When the waves reach your friend, they bounce back to you!
   Light waves travel 300,000 kilometers (186,000 miles) per second! They can also travel through a vacuum. That is why light from the sun and distant stars can travel through space to the earth!

Useful Words and Expressions;
1. destructive破坏的
2. seismic地震的
3. vacuum真空