【內容提示】
以“Sky”爲題寫一篇描寫文。內容要點如下：
①天氣晴朗時仰望天空，地球好像被一個藍色的大圓頂所覆蓋。
②天空之所以呈藍色，是因爲陽光射入地球大氣層時起了變化。
③陽光與微小的空氣分子碰撞，便散射開來。散射開的光有多種顏色，這些顏色的組合便是蔚藍色。
④有時日落時天空呈紅色。這時的陽光在天空中以較低的角度射來。
⑤太陽在天空下落時，其光線穿過更多的空氣，於是波長較長的橙色和紅色也散射開來。日落時，波長較短的藍色光不能穿透大氣層，所以藍光就看不見了。
⑥站在一塊地勢平展的地方極目四望，天空好像處處都與地球交接。交接處叫地平線。
⑦無論你往哪個方向看，地平線與你的距離總是等長。
⑧無論你在何地，地球表面總是向下彎的，彎向遠方；那是人們目力不及之處，這便是地平線。
【作文示範】
Sky
When you look at the sky on a clear day， it seems as if the earth is covered by a blue dome①。 The sky is blue because of what happens to sunlight as it enters the earth＇s atmosphere。 Sunlight is made up of light rays of all lengths。 The longer rays make up red light and the shorter rays make up blue light。 As sunlight enters the earth＇s atmosphere， it strikes the tiny molecules ② of air and is scattered③。 The scattered light is made up mostly of violet④， blue， a little green， and a little yellow and red。 The combination of these colors is sky blue。
Sometimes at sunset the sky looks red。 The sunlight is coming toward you from a lower angle in the sky。 The lower the sun， the longer is the path that the light rays must travel through the air。 The shorter blue rays are scattered immediately when striking the molecules of air。 The sunlight that you see lacks the blue and violet rays that the air has scattered。 As the sun gets lower in the sky， the light rays must pass through more air，and so the longer orange and red rays also become scattered。 At sunset， the shorter blue rays never get through the atmosphere， so you do not see them。
If you stand at a place where the surface of the earth is flat， the sky seems to meet the earth in any direction you look。 The point at which the sky and the earth seem to“meet”is called the horizon。 The horizon is at the same distance from you， no matter in what direction you look。 If you walk toward the horizon， you do not get closer to it。 No matter where you are， the surface of the earth curves ⑤ downward and away from you。 The distance at which you can no longer see the earth＇s surface----because it has curved down too far----is the horizon。
【詞語解釋】
①dome [d+um] n。 圓蓋；穹窿
②molecule [＇m&likju:l] n。 分子
③scatter [＇sk$t+] v。 使消散；使分散
④violet [＇vai+lit] n。 紫色；紫羅藍色
⑤curve [k+:v] v。 成曲形；彎曲
【寫法指要】
這是一篇科技知識小品式的天象描寫文。作者以從上到下的空間順序安排結構，先寫蒼穹的蔚藍，用一個帶有明喻“as if”的句型開頭，寫出天空像一個藍色的大圓蓋，然後以光波長短來分析天空呈藍色的道理。接着作者寫日落時天空呈紅色的情景：這時的'陽光在天空中以較低的角度射來，太陽越低，光線穿過空氣的路程就越長；波長短的藍色碰撞空氣分子後立即散射，因此人們看不見，人們看到的是波長較長的橙色和紅色。最後，作者寫人們站在平地上極目四望的情景：人們看到的是天地相接處的地平線。這樣，作者通過上、中、下三個層面的描寫，給人們描繪出了天空呈藍色、紅色以及地平線形成的科學道理。
