The Speed of Light

叶欣

Light travels fast.

You can get some idea of this difference by watching the start of a race. If you stand some distance away from the starter， you can see smoke from his gun before the sound reaches your ears.

This great speed of light produces some strange facts.

Sunlight takes about 8 minutes to reach us. If you look at the light of the moon tonight， remember that the light rays① left the moon 1.3 seconds before they reached you. The nearest star is so far away that the light that you can see from it tonight started to travel towards you four years ago at a speed of nearly 2 million km. per minute. In some cases， the light from one of tonights stars had started on its journey to you before you were born.

Thus， if we want to be honest， we cant say， “The stars are shining tonight.” We have to say instead， “The stars look pretty. They were shining four years ago， but their light has only just reached the earth.”

Choose the best answer.

1. Which travels faster？

A. Sound.         B. Heat.                 C. Radio wave （电波）.         D. Light.

2. If you stand 200 meters away from a man who is firing a gun （開枪） to start a race， you will find out that ____.

A. you can hear the sound before you see the smoke

B. you can see the smoke before you hear the sound

C. the sound will reach you before the man fires

D. the sound is as fast as the light

3. Sunlight ____ than the light of the moon.

A. has to travel a greater distance        B. moves less quickly

C. travels much more quickly                D. is less powerful （有力的）

4. The scientific way of saying “The stars are shining tonight” should be ____.

A. the stars have been shining all the time

B. the stars seen tonight will shine four years later

C. the stars were shining long ago but seen tonight

D. the starlight seen today could be seen four years ago