by Suzanne Elvidge BSc

翻譯：思葦

科學(xué)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(lán)色的？”這個(gè)問(wèn)題幾乎困惑了我整個(gè)童年。父母不知道這個(gè)問(wèn)題的答案。母親在我的追問(wèn)下，對(duì)我說(shuō)：“媽媽不知道哦，你好好讀書(shū)，以后學(xué)會(huì)了告訴媽媽好嗎？”嗯，我有一個(gè)好媽媽。（喂，跑題了?。┡d沖沖上學(xué)堂，結(jié)果書(shū)本上也沒(méi)有教，又沒(méi)膽去問(wèn)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(zhǎng)]. 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(jìn)行傳播，不同的顏色具有不同的波長(zhǎng)。物體顯出特定的顏色是因?yàn)樗鼈儠?huì)對(duì)這個(gè)顏色進(jìn)行反射或者散射——葉子之所以看起來(lái)是綠色的，是因?yàn)樗瓷渚G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(guò)]. 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(zhì)]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(lán)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(gè)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(duì)面，觀察透過(guò)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(guò)去。透進(jìn)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?jǐn)嚢瑁儆檬蛛娡舱丈溥^(guò)去——容器中的光和照在紙上的光發(fā)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(dá)爾效應(yīng)”（想一想汽車前燈在大霧天如何將光反射出來(lái)——霧正是由懸浮在空氣中的小水滴形成的）。

光可以直接透過(guò)水進(jìn)行傳播，沒(méi)有經(jīng)過(guò)什么反射和散射，因而顏色不會(huì)發(fā)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(huì)讓瓶子或杯子里的光微微發(fā)藍(lán)，而投射在紙上的光則會(huì)顯出帶紅的色調(diào)。

在手電筒所照射出來(lái)的光線中，比起紅色、橙色和黃色的部分（波長(zhǎng)較長(zhǎng)），藍(lán)色的部分（波長(zhǎng)較短）更常被懸浮粒子反射和散射。當(dāng)這種散射光傳遞到我們的眼睛時(shí)，就會(huì)讓這杯混合物看上去顯藍(lán)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(guò)，所以照在紙上的光就會(huì)帶有這幾種顏色。

在杯子或瓶子中心的光看上去也會(huì)微微發(fā)紅——這是因?yàn)楹驼赵谌萜鬟吘壍墓獗绕饋?lái)，中心光線要穿過(guò)更多粒子，所以更多藍(lán)光會(huì)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(lán)天的奧秘

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

行政公益訴訟回顧與展望—基于“一決定三解釋”及試點(diǎn)期間相關(guān)案例和《行政訴訟法》修正案的分析…黃學(xué)賢（41）

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(fā)]refect the red and orange light.

在地平線附近的光線要穿過(guò)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(fā)白。

日出與日落

白天的時(shí)候，太陽(yáng)光自上而下照射下來(lái)，所以它要穿過(guò)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(yáng)的光線由近地平線照射出來(lái)，要穿過(guò)更為大量的大氣進(jìn)行傳播，于是更多藍(lán)光和綠光會(huì)被散射到各個(gè)方向。等到這道光線到達(dá)人們的眼睛時(shí)，大部分的藍(lán)光和綠光已經(jīng)消失，只剩下紅光、橙光和黃光，讓天空和云彩呈現(xiàn)出紅色、橙色和黃色。

丁達(dá)爾效應(yīng)還會(huì)帶來(lái)色彩斑斕的日出與日落——水蒸氣和污染或者火山噴發(fā)帶來(lái)的顆粒會(huì)對(duì)紅光和橙光進(jìn)行反射。

約翰·丁達(dá)爾（John Tyndall，1820—1893），英國(guó)物理學(xué)家，英國(guó)皇家學(xué)會(huì)物理學(xué)教授，著名物理學(xué)家、化學(xué)家邁克爾·法拉第的學(xué)生和朋友。1869年，丁達(dá)爾發(fā)現(xiàn)當(dāng)一束光線透過(guò)膠體，從入射光的垂直方向可以觀察到膠體里出現(xiàn)一條光亮的“通路”，而溶液則沒(méi)有這條“通路”，這種現(xiàn)象后來(lái)被稱為“丁達(dá)爾現(xiàn)象”，也叫“丁達(dá)爾效應(yīng)”（Tyndall Effect）。丁達(dá)爾效應(yīng)是區(qū)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(guó)埃塞克斯郡莫爾登（Malden）的朗弗德林園。瑞利以嚴(yán)謹(jǐn)、廣博、精深著稱，并善于用簡(jiǎn)單的設(shè)備做實(shí)驗(yàn)獲得十分精確的數(shù)據(jù)。他是在19世紀(jì)末達(dá)到經(jīng)典物理學(xué)顛峰的少數(shù)學(xué)者之一，在眾多學(xué)科中都有卓越成果。他在聲學(xué)、波的理論、光學(xué)、光的散射、電力學(xué)、電磁學(xué)、水力學(xué)、液體流動(dòng)理論方面都做出了不可磨滅的貢獻(xiàn)，其中尤以光學(xué)中的瑞利散射和瑞利判據(jù)、物性學(xué)中的氣體密度測(cè)量方面的影響最為深遠(yuǎn)。1904年，他因和拉姆塞同時(shí)發(fā)現(xiàn)了惰性元素氬（Ar）而榮獲該年度的諾貝爾物理學(xué)獎(jiǎng)。

Know More