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#### Electromagnetic Waves

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What does the professor mainly discuss?
• A. Methods of converting radio waves into sound waves

• B. Features of different types of electromagnetic radiation

• C. The various paths that very-low-frequency waves follow on Earth

• D. The emission and detection of very-low-frequency waves

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NARRATOR:Listen to part of a lecture in a physics class. The professor has been discussing electromagnetic waves.

MALE PROFESSOR:So are there are any questions before we continue our discussion of different types of electromagnetic waves?Um, today we'll focus on radio waves, and specifically, very low frequency radio waves. Ah Yes, Tim?

MALE STUDENT:Are you going to talk at all about the difference between radio waves and sound waves?

MALE PROFESSOR:Um, OK. That might be a good place to start, actually. Sound waves are mechanical in nature, right?Ah, they can only originate and spread in places where there is some dense physical medium, like atmosphere, or water.Ah, they result from changes in pressure in that medium, like changes in air pressure.So they can't travel through a vacuum, where there's no dense physical medium. Which is why they can't travel through interplanetary space.Radio waves, on the other hand, are fundamentally different from sound waves.They are electromagnetic: they result from oscillations of the electromagnetic field and don't need a physical medium.So they, like other types of electromagnetic wave, can travel basically anywhere— through a vacuum or through atmosphere or water.

Now, radio waves can be detected.For example, very low frequency radio waves can be detected with a special type of radio receiver called a very low frequency, radio, or VLF radio.Which can pick up radio waves with very low frequencies , from 3 to 30 kilohertz, which aren't really picked up by a regular household or car radio.So VLF radios pick up VLF radio waves and convert them to sounds we can hear.

Um, on Earth the main source of naturally occurring VLF emissions is lightning, which generates a pulse of radio waves every time it flashes. Ah yes, Laura?

FEMALE STUDENT:Since you almost always get lightning with thunderstorms, we can pick up VLF waves pretty often, right?You just have to wait until there's a thunderstorm...

MALE PROFESSOR:[to suggest “think again”] Ah, do you? Have to wait? VLF receivers are very sensitive and VLF mwaves travel very far. So we can pick up emissions from lightning that's far away.So, actually, you can pretty much listen to them all the time, because lightning strikes Earth constantly, about a hundred times per second.Even if there's no lightning where you are, with a VLF radio you can hear the crackling from storms that are thousands of kilometers away.However, some times of day are better than others for picking up VLF waves. Daytime isn't as good as nighttime, for example.And what's more, my colleague Dennis Gallagher says—and in my opinion, he's right—he says the best time to listen for them is around sunset or sunrise.That's when there are natural waveguides in the local atmosphere.

FEMALE STUDENT:Did you say waveguide?

MALE PROFESSOR:Yes. A waveguide, usually it refers to a device, like a metal conductor, that's used to guide and direct waves.But waveguides also occur naturally—they make a path for radio waves to follow in our atmosphere.These natural waveguides occur when the Sun is rising or setting, which makes sunrise and sunset good times to pick up VLF emissions.Now, there are a few different sounds that you can hear on a VLF receiver, because when lightning strikes, the radio waves travel different distances and in different ways before they reach the receiver.Some really interesting ones are called "whistlers."

Whistlers come from lightning-generated radio waves that leave Earth's atmosphere and travel into Earth's magnetosphere before bouncing back down.Not all radio waves do this. And the sound they make... well we call them "whistlers" because they sound like slowly descending tone... and no two whistlers are alike—uh, to me they're the most intriguing.Ah, another interesting sound is the "tweek."

Tweeks are the result of VLF waves that have traveled a long distance through the waveguides.They produce a chirpy sound because the higher-frequency parts of the wave reach the radio receiver before the lower-frequency parts.The entire wave is still considered very low frequency—it's just that some parts of the wave have lower frequencies than others. OK?

• 旁白：请听物理学课上的部分内容。教授正在讨论电磁波。

教授：在我们继续讨论不同类型的电磁波之前，你们有什么问题吗？今天我们的重点是无线电波，具体来说是非常低频的无线电波。Tim，请说。

学生：你会讲无线电波和声波之间的区别吗？

教授：好的，事实上，这也许是个不错的切入点。声波本质上属于力学，对吧？它只能在有一些密实的物理介质的环境中发生和传播，比如空气和水。它们由介质中压强的变化而产生，比如气压的变化。所以它们不能在真空环境中传播，那里没有密实物理介质。这也就是为什么它们无法穿过行星间的太空了。而无线电波和声波有着本质的区别。它们是电磁的。它们由电磁场的振动中产生，不需要物理介质。所以它们，和其他类型的电磁波一样，基本上可以在任何地方穿梭，可以穿过真空环境或者空气或水。

无线电波是可以探测到的。比如说，很低频次的无线电波可以用一种特别类型的无线电接收器探测到，这种接收器叫做非常低频次的无线电接收装置或VLF无线电装置。它能接收到低频次的无线电波，从3千赫到30千赫，这个范围内的无线电波不会被寻常家用的或车载无线电装置接收到。所以VLF无线电装置会接收到VLF无线电波，并且把它们转变成我们能听到的声音。

在地球上，自然释放VLF的主要来源是闪电，它每一次闪现时会产生一个脉冲的无线电波。Laura，请讲。

学生：既然在雷暴中几乎总是会有闪电，那我们可以经常接收到VLF电波，对吗？你只需要等到有雷暴就行了。

教授：是吗？要等吗？VLF接收器非常敏感，而且VLF电波传播得非常远。所以你从很远的地方就能接收到闪电释放的电波。所以事实上，你几乎一直都在听这些电波，因为地球上不断有闪电发生，大概每秒一百次。即使你所在的地方没有闪电，但是有了VLF接收器，你能听到几千公里以外的暴风雨产生的动静。但是，一天中有些时候比其他时间更适合接收VLF声波。比如白天不如晚上。而且，我的同事Denis Gallagher说，就我看来他说的很对。他说听这些声波最好的时间是在日落或日出的时候。那是当地空气中的自然波导出现的时候。

学生：你刚才说了波导吗？

教授：是的，波导。通常它指的是一个装置，比如用来引导和控制电波的金属导体。但是波导也会天然产生。它们为无线电波在我们的大气中开辟了一条遵循的轨道。太阳升起或落下的时候这些天然的波导就会产生，这就使得日落或日出成为了接收VLF发射的好时机。在一个VLF接收器上能听到好几种不同的声音，因为出现闪电时，无线电波在到达接收器之前会经过不同的距离和不同的路径传播。一些非常有趣的电波叫做啸声信号。

啸声信号来自闪电产生的无线电波，这种电波离开了地球的大气层，在重新反弹回来之前传播进了地球的磁气圈。不是所有的无线电波都会这么做，而它们发出的声音，我们叫它们啸声信号是因为它们听起来像缓慢下降的调子。任何两个啸声信号都是不同的。对我来说它们是最有趣的。另一个有趣的声音是吱声。

吱声是VLF电波的产物，这些电波借助波导穿过了很长的距离。它们发出了一种活泼的声音，因为这种电波较高频次的部分到达无线电接收器比低频次的部分更早。但这整段电波仍被认为是非常低频次的电波。只是电波中的某些部分的频次比其它的要低。知道了吗？

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题型分类：主旨题

原文定位：文章开头无信号

选项分析：教授一开始就说了要谈 low frequency radio waves ，然后学生问了要讲sound wave radio wave的不同吗？教授说就开始讲不同。讲完不同之后教授开始讲怎么去detect，用VLFdetect，所以选D文章一直讲的都是low frequency waves，只有在最后提了两种不同的电磁波whistlerstweeks（注意sound wave不是电磁波）选项B错误

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