英語科普文選教程第一冊第4課

Light
Light travels very fast. It moves at 300,000 kilometers a second. Light reaches us from the moon in less than a second and a half. The moon is about 382,000 kilometers away.
Light from the sun reaches us in eight and a half minute. The sun is about 149,640,000 kilometers form the earth.
The other stars are farther away than the sun. Light from the nearest star reaches us in about four years. When you look at the star you do not see its present condition. You see it as it used to be. Light form some stars takes hundreds of years to reach us. We never see a star as it is now. We see it as it was long ago, perhaps hundreds or thousands of years ago.
Astronomers watch the stars through big telescopes. Some of these telescopes have glass lenses in them. A lens is a round piece of glass, but it is not flat. The side of a lens is curved. The middle part of some lenses is thicker than the edge. In other lenses, the edge is thicker than the middle part.
Cameras have lenses at the front. A very good lens costs a lot of money. It costs more than any other parts of the camera.
A ray of light usually travels in a straight line. But sometimes it bends. The light reaches the film of the camera through the lens. It leaves the air and goes into the glass; then it bends. The ray also bends when it leaves the glass. Light also bends when it reaches the lens of a telescope.
Some telescopes do not use a lens at the front. They use a curved mirror at the back. A curved mirror is better than a lens in some ways.
The world’s biggest telescope is in the Soviet Union. The next biggest is on Mount Palomar in the U.S. Near it, on Mount Wilson, there is another biggest telescope. All three of them use mirrors, not lenses at the front. With these great telescopes astronomers can see stars and other things very far away. Some of these things are not only stars. They are great groups of stars.
The sun and the earth are in the great group of stars. There are about 100,000,000,000 stars in it. We call this group the galaxy. Outside the galaxy there is empty space. But thousands of millions of kilometers away, there is another galaxy. Light form this other galaxy reaches us after about two million years. There are millions of these galaxies, and they appear to be rushing away from us.
The astronomers at Mount Palomar and Mount Wilson and other observatories can see some of them well. But they cannot see one as it is now. The light takes millions of years to arrive here. So they see a distant galaxy as it used to be. The light left it millions of years ago. It traveled across space and then went into an astronomer’s eye. Perhaps no men were living when it started.