在 2019 年 12月 8日的托福考试中出现了关于 moonbow(月虹)的物理学讲座文章，相比较艺术，生物，天文这些ETS青睐的高频话题来说，虽然物理类所占比例较少，但是在改革后还是需要引起警惕。本期学科知识就带大家探讨下关于moonbow的背景知识及相关物理学和天文学拓展。
一. 相关背景补充
A moonbow (also known as a lunar rainbow or white rainbow) is a rainbow produced by moonlight rather than direct sunlight. Other than the difference in the light source, its formation is the same as for a solar rainbow: It is caused by the refraction of light in many water droplets, such as a rain shower or a waterfall, and is always positioned in the opposite part of the sky from the moon relative to the observer.
Moonbow的成因
由于月亮本身不发光，它的光是反射太阳光来的，因此形成月虹的其实还是太阳光，并且和太阳光一样，月光也是由各种颜色的光混合而成，在雨后，大量的小水滴悬浮在空中，月光通过时发生了折射，在折射过程中，不同颜色的光折射角度发生改变，便形成了七色的光谱。但月光毕竟是月球反射太阳光而形成，因此月光远远没有阳光明亮，月虹的色彩也相应的没有那么明显，所以月虹一般只能分辨出一两种颜色。当然，也不一定非要雨后才能看见，只要空气中有大量水汽，比如瀑布旁边，月光又足够明亮，也有可能看到月虹。
moonbow的姐妹： rainbow （彩虹）
A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc.Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Rainbows can be full circles.
Halo （光晕）
A circular band of colored light around a light source, as around the sun or moon, caused by the refraction and reflection of light by ice particles suspended in the intervening atmosphere.
区别：在天气变化以前，人们有时会在月亮的周围，看到一个甚至两个以上的彩色或白色光圈，而且太阳光和月亮光也似乎暗淡了许多。人们把太阳或月亮周围出现的这种光圈叫“晕”。太阳周围出现的光圈叫日晕，月亮周围出现的光圈叫月晕，这是一种比较常见的气象现象。月晕是月光通过云层中的冰晶时，经折射而成的光在月亮周围形成的白色或彩色光圈。而月虹则是在月亮反方向的天空中。
观测景点
全世界目前最有名的两处月虹景点，一处是位于美国肯塔基州的坎博兰瀑布，另一处是非洲赞比亚和津巴布韦之间的维多利亚瀑布。除此二处之外，美国优胜美地国家公园在瀑布区也常有观测到月虹的纪录。
维多利亚瀑布位于非洲南部赞比亚和津巴布韦接壤区域，在赞比西河上游和中游交界处，是世界上、最美丽和最壮观的瀑布之一，宽1,700多米，处达108米。一条深邃的岩石断裂谷横切赞比西河，滔滔河水流经此处时骤然下落，形成维多利亚瀑布。巨量的河水下落时，如万雷轰鸣，动地惊天，溅起的白色水雾，有如片片白云和轻烟在空中缭绕。因此维多利亚瀑布也被称为“雷鸣之烟”或“轰轰作响的烟雾”（the smoke that thunders）。每逢满月升起，水雾折射月光，就形成了月虹奇观，景色十分迷人。
二．学科话题拓展（TPO真题）
1） Moon
Clementine's data indicated that the wall of a south polar crater was more reflective than expected. So some experts think there's probably ice there. Also, data from a later mission indicate significant concentrations of hydrogen, and by inference, water, less than a meter underground at both poles.
MALE STUDENT: If there's water, how'd it get there? Underground rivers?
MALE PROFESSOR: We think meteors that crashed into the Moon, or tails of passing comets, may have introduced water molecules. Any water molecules that found their way to the floors of craters near the Moon's poles, that water would be perpetually frozen because the floors of those craters are always in shadow. Uh, furthermore,if the water ice was mixed in with rock and dust, it'd be protected from evaporation.
FEMALE STUDENT: So, are you saying there might be primitive life on the Moon?
MALE PROFESSOR: Uh, that's not my point at all! Um, OK, say there is water ice on the Moon. That would be of very practical value for a future Moon base for astronauts. Uh, water ice could be melted and purified for drinking. It could also be broken down into its component parts—oxygen and hydrogen. Oxygen could be used to breathe. And hydrogen could be turned into fuel, rocket fuel. So, water ice could enable the creation of a self-sustaining Moon base someday, a mining camp, perhaps, or, uh, a departure point for further space exploration.（TPO5 L2 Moon Landing）
月球上可能会存在水（撞击月球的流星或经过月球的彗星会留下一些水分子）。如果这些水和岩石尘土混在一起可以避免蒸发。
2) Spectrum
You all know how when you take a crystal prism and pass a beam of sunlight through it, you get a spectrum which looks like a continuous band of rainbow colors. The light that we see with our human eyes as a band of rainbow color falls in the range of what's called visible light.
so we've got a spectrum of a beam of sunlight and it looks like the colors bleed into each other, uh, there're no interruptions, just a band flowing from violet to green to yellow to…you get the idea. Well what happens if the sunlight spectrum is magnified? [pause - then slightly under breath] Maybe you all didn't do the reading. Well here's what you'd see:I want you to notice that this spectrum is interrupted by dark lines, called spectral lines.
If you really magnified the spectrum of the sunlight, you could identify more than a hundred thousand of 'em. They may look kinda randomly placed, but they actually form many distinct patterns.（TPO3 L4 Spectroscopy）
太阳光柱透过棱镜会得到一条彩虹色彩斑带。这条光谱看起来像是色彩都相互融合。但是其实如果放大光谱来看发现这些光带被叫作光谱线的黑色线所阻隔，而且每个光谱线看上去像是随意排列但是其实组成了不同的图案。
3) Reflection
a cloud either contributes to the cooling of Earth's surface or to its heating.Earth's climate system is constantly trying to strike a balance between the cooling and warming effects of clouds.
It's very close, but overall, the cumulative effect of clouds is to cool Earth rather than heat it. And this balance between the amount of solar radiation—energy from the Sun—that's absorbed by Earth and the amount that's reflected back into space, we call this Earth's "radiation budget."And one way we keep track of the radiation budget is by looking at the albedo of the different surfaces on the planet. （TPO23 L2 Earth radiation budget）
云的累积作用是降低地球的温度而不是加热。这个平衡是指太阳的辐射量，也就是被地球吸收太阳能量与反射回宇宙中的能量之间的平衡。我们观察辐射收支的方法之一就是通过观测地球不同表面的反照率。
三．话题词汇补充：
moonbow 月虹
halo /'heɪloʊ/ 光晕
reflection /rɪ'flekʃ(ə)n/ 反射
refraction /rɪ'frækʃ(ə)n/ 折射
radiation /.reɪdi'eɪʃ(ə)n/ 辐射
optical /'ɑptɪk(ə)l/ 视觉的
albedo /æl'bɪdoʊ/ 反射率
droplet /'drɑplət/ 小滴
fall 瀑布
accumulate /ə'kjumjə.leɪt/ 累积
cumulative /'kjumjələtɪv/ 累积的
multicolored /məltiˈkələrd/ 多色的
spectrum /'spektrəm/ 光谱
spectroscopy 光谱学
prism /'prɪzəm/ 棱镜
beam /bim/ 光束
四．总结
听力考试改革之后，除了常考的艺术，天文，生物心理话题，ETS也会不定期的加入一些新型话题，物理和化学以及环境科学类讲座出现频率也在上升。所以对于想要听力拿高分的同学来说，平时在练习高频专题讲座的基础上也要多多关注物理化学类学科知识和讲座，确保万无一失。
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