科學知多少：到底天空是什么顏色？

by Suzanne Elvidge BSc

翻譯：思葦

科學知多少：到底天空是什么顏色？

Science Project: What Colour Is the Sky?

by Suzanne Elvidge BSc

翻譯：思葦

“天空為什么是藍色的？”這個問題幾乎困惑了我整個童年。父母不知道這個問題的答案。母親在我的追問下，對我說：“媽媽不知道哦，你好好讀書，以后學會了告訴媽媽好嗎？”嗯，我有一個好媽媽。（喂，跑題了！）興沖沖上學堂，結果書本上也沒有教，又沒膽去問老師，就這么不了了之了。童鞋們，你知道天空是什么顏色嗎？

White light is made up of a mixture[混合]of different colours. Light travels in waves, and different colours have different wavelengths[波長]. Things seem a particular colour because of the light they refect[反射]or scatter[散射]—a leaf appears green because it reflects green light.

白光是由不同顏色混合而成的。光以波的形式進行傳播，不同的顏色具有不同的波長。物體顯出特定的顏色是因為它們會對這個顏色進行反射或者散射——葉子之所以看起來是綠色的，是因為它反射綠光。

Scattering Light

Shine a torch[手電筒]through an empty glass or bottle and look at the colour of the light in the bottle. Hold a piece of paper on the other side of the bottle and look at the colour of light that has passed through[通過]. Fill the glass or bottle with water and shine the torch through it. What colour is the light passing in the bottle or glass,and what colour is the light on the paper? Add a drop or two of milk and mix it thoroughly[充分地], and shine the torch through it again—what happens to the light in the glass or bottle, and the light on the paper?

Particles in suspension[懸浮]reflect light—this is called the “Tyndall Effect” (think of how car headlights[車頭燈]reflect off fog, which is a suspension of water droplets[微水滴]in air).

Light travels directly through water, with little reflection and scattering, and the colour does not change. Milk in water is a suspension of fat and protein[蛋白質]particles[微粒]in water. The suspended particles should change the colour of the light in the bottle or glass to a bluish[帶藍色的]tinge[淡色], with the light on the paper having a reddish[微紅的]tinge.

The blue part of the light (shorter wavelength) from the torch should be refected and scattered by the particles in the suspension, more than the red, orange and yellow parts of the light (longer wavelength). When this scattered light reaches people’s eyes, it makes the mixture seem blue, particularly at the edges. The particles allow more of the red, orange and yellow light through, so the light on the paper will have more of these colours in it.

The light at the centre of the glass or bottle may also seem to have a reddish tinge—this is because it has passed through more particles than the light at the edges, so more of the blue light has been scattered away.

淺談光散射

用手電筒照射一個空杯子或者空瓶子，觀察瓶子里的光是什么顏色。將一張紙放在瓶子對面，觀察透過瓶子的光是什么顏色。在杯子或瓶子里裝滿水，用手電筒照射過去。透進瓶子或杯子里的光是什么顏色？照在紙上的光又是什么顏色呢？加入一兩滴牛奶，充分攪拌，再用手電筒照射過去——容器中的光和照在紙上的光發生了什么變化呢？

懸浮的粒子能夠反射光，這就是所謂的“丁達爾效應”（想一想汽車前燈在大霧天如何將光反射出來——霧正是由懸浮在空氣中的小水滴形成的）。

光可以直接透過水進行傳播，沒有經過什么反射和散射，因而顏色不會發生改變。水中的牛奶就是懸浮的脂肪和蛋白顆粒。這些懸浮粒子會讓瓶子或杯子里的光微微發藍，而投射在紙上的光則會顯出帶紅的色調。

在手電筒所照射出來的光線中，比起紅色、橙色和黃色的部分（波長較長），藍色的部分（波長較短）更常被懸浮粒子反射和散射。當這種散射光傳遞到我們的眼睛時，就會讓這杯混合物看上去顯藍色，在杯子邊緣尤為明顯。這些粒子能讓大部分的紅光、橙光和黃光通過，所以照在紙上的光就會帶有這幾種顏色。

在杯子或瓶子中心的光看上去也會微微發紅——這是因為和照在容器邊緣的光比起來，中心光線要穿過更多粒子，所以更多藍光會被散射出去。

Blue Skies

Air molecules[微粒]also affect light. They absorb[吸收]the light and then release[釋放](scatter) it in another direction. This is called the “Rayleigh Effect”. Air molecules absorb and release blue light more often than red, orange and yellow light. As this light is scattered, it reaches people’s eyes, making the sky seem blue. The red, orange and yellow light carries straight on without being scattered by the air molecules, so people do not see as much of it.

藍天的奧秘

空氣中的微粒對光線同樣會產生影響。它們能夠吸收光，然后將其向著另外一個方向釋放（散射）出去，這被稱為“瑞利效應”。這些微粒所吸收與釋放的更多是藍光，而不是紅光、橙光和黃光。這道經過散射的光線傳遞到人們的眼睛，就會讓天空看起來顯藍色。而直接穿過去的紅光、橙光和黃光由于沒有被空氣微粒所散射，所以人們很難接收到這樣的光線。

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Light from the horizon passes through more air, so it’s more likely to be scattered away, making the sky there seem paler[蒼白的]in comparison[相比之下].

Sunrise and Sunset

In the day, light from the sun comes from above, so it passes through less air, and has been scattered by fewer air molecules. In the morning and evening, light from the sun comes from near the horizon, so it travels through a lot more of the atmosphere[大氣層], and more of the blue and green light is scattered in different directions. By the time the light reaches people’s eyes, a lot of the blue and green light has been lost, and only the red, orange and yellow light is left, making the sky and clouds look red, orange and yellow.

The Tyndall Effect also creates brightly coloured sunrises and sunsets, when water vapour[水蒸氣]and particles from pollution[污染物]or volcanic eruptions[火山爆發]refect the red and orange light.

在地平線附近的光線要穿過更多空氣，因而更容易被散射出去，這就讓那里的天空相比之下顯得更加發白。

日出與日落

白天的時候，太陽光自上而下照射下來，所以它要穿過的空氣比較少，受到空氣微粒的散射也比較少。而在早上和傍晚，太陽的光線由近地平線照射出來，要穿過更為大量的大氣進行傳播，于是更多藍光和綠光會被散射到各個方向。等到這道光線到達人們的眼睛時，大部分的藍光和綠光已經消失，只剩下紅光、橙光和黃光，讓天空和云彩呈現出紅色、橙色和黃色。

丁達爾效應還會帶來色彩斑斕的日出與日落——水蒸氣和污染或者火山噴發帶來的顆粒會對紅光和橙光進行反射。

約翰·丁達爾（John Tyndall，1820—1893），英國物理學家，英國皇家學會物理學教授，著名物理學家、化學家邁克爾·法拉第的學生和朋友。1869年，丁達爾發現當一束光線透過膠體，從入射光的垂直方向可以觀察到膠體里出現一條光亮的“通路”，而溶液則沒有這條“通路”，這種現象后來被稱為“丁達爾現象”，也叫“丁達爾效應”（Tyndall Effect）。丁達爾效應是區分膠體和溶液的一種常用物理方法。

瑞利，原名約翰·威廉·斯特拉特（John William Strutt），尊稱瑞利男爵三世（Third Baron Rayleigh），1842年11月12日出生于英國埃塞克斯郡莫爾登（Malden）的朗弗德林園。瑞利以嚴謹、廣博、精深著稱，并善于用簡單的設備做實驗獲得十分精確的數據。他是在19世紀末達到經典物理學顛峰的少數學者之一，在眾多學科中都有卓越成果。他在聲學、波的理論、光學、光的散射、電力學、電磁學、水力學、液體流動理論方面都做出了不可磨滅的貢獻，其中尤以光學中的瑞利散射和瑞利判據、物性學中的氣體密度測量方面的影響最為深遠。1904年，他因和拉姆塞同時發現了惰性元素氬（Ar）而榮獲該年度的諾貝爾物理學獎。

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