科學知多少：到底天空是什么顏色？

“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 reflect[反射] or scatter[散射]—a leaf appears green because it reflects green light.

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 reflected 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.

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.

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] reflect the red and orange 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。

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。……