Ｗｈｙ ｄｏｅｓ ｔｈｅ ｓｋｙ ｃｈａｎｇｅ ｃｏｌｏｒｓ？

Is the sky blue?The sky has no color.We know that there is air around the world.When planes fly，they need air to lift their wings.Planes can’t fly very high because the air gets thinner when they go higher.If we go up about 1200 kilometers from the earth，we’ll find no air there.

When we talk about the sky，we usually mean the atmosphere around the earth.It is like a very thick“cover” over the earth and keeps it warm.Without the “cover”，the earth would be a very cold place and nothing could live on it.

Looking up above us，you will see the blue sky with white clouds.It is usually light blue，but the sky would be dark blue if you go to the top of the highest mountains.And you will see fewer clouds between you and the sun.

And if you sit in a spaceship high above the earth，there is no air at all.The sky would be so dark that its color would be black.

Sometimes the sky doesn’t look blue.When the sun rises and falls，you will see red，orange，and other colors in the sky.These are the most beautiful moments of the day.

Now you may understand that we see colors because the air scatters the sunlight.Light travels through the air.Without air，there would be no light and colors.

譯文：

天為什么會變換顏色？

天空是藍的嗎？天空是沒有顏色的。我們知道這個世界的周圍是空氣。當飛機飛行的時候，需要空氣來托起它們的翅膀。飛機不能飛得太高，因為當它們飛到一定的高度時，空氣會變得更加稀薄。如果我們從地面上升到1200千米的高度時，我們會發現那里沒有空氣。

當我們談起天空時，通常指的是地球外面的大氣層。它像一床非常厚的“被子”蓋在地球上，使之保持溫暖。沒有這個“被子”，地球將會是一個非常寒冷的地方，地面上會寸草不生。

抬頭看看我們的天空，你會看到藍天白云。平日里，它是明亮的藍色。但是如果你到高山頂上去看，你會發現天空會是深藍色的。并且你將會看到你和太陽之間的云彩變少了。

如果你乘宇宙飛船去很高的空中，你會發現那里一點空氣都沒有。那里的天空非常暗，它的顏色看起來也是黑色的。

有時候天空不顯現藍色。當太陽升起或落下時，你可以看到天上有紅、桔黃和一些其他的顏色。這是一天中最美麗的時刻。

現在你也許明白了，我們看到色彩是因為空氣折射了太陽的光。光是在空氣中傳播的，沒有空氣，也就沒有了光明和色彩。

Exercises:

1.The sky usually looks light blue because________.

A.light is light blue

B.the air is light blue

C.sunlight is light blue

D.the air scatters some blue color from the sunlight

2.When ______，the sky looks dark blue.

A.there is no air between you and the sun

B.you are on the highest mountains

C.there is a high mountain near you

D.the mountains around you are very high

3.If ______，the color of the sky would be black.

A.you are in a spaceship

B.the spaceship is high in the sky

C.there is no air to scatter the sunlight

D.it is snowing

KEYS:1.D2.B3.C