Electromagnetic Waves托福听力原文翻译及问题答案

Electromagnetic Waves托福听力原文翻译及问题答案

一、Electromagnetic Waves托福听力原文：

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: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?

二、Electromagnetic Waves托福听力中文翻译：

旁白：在物理课上听一节课的一部分。教授一直在讨论电磁波。

男教授：在我们继续讨论不同类型的电磁波之前，还有什么问题吗？嗯，今天我们将重点讨论无线电波，特别是极低频无线电波。啊，是吗，蒂姆？

男学生：你打算谈谈无线电波和声波的区别吗？

男教授：嗯，好的。实际上，这可能是一个很好的开始。声波本质上是机械波，对吗？啊，它们只能在有致密物理介质的地方产生和传播，比如大气或水。啊，它们是由介质中压力的变化引起的，就像空气压力的变化一样。所以它们不能穿过真空，因为那里没有致密的物理介质。这就是为什么他们不能穿越行星际空间。

另一方面，无线电波与声波有着根本的不同。它们是电磁的：它们是电磁场振荡的结果，不需要物理介质。因此，就像其他类型的电磁波一样，它们基本上可以通过真空或大气或水传播到任何地方。

现在，可以检测到无线电波。例如，可以使用一种称为甚低频、无线电或VLF无线电的特殊类型的无线电接收器来检测极低频无线电波。它可以接收频率非常低的无线电波，从3千赫兹到30千赫兹，而普通的家用或车载收音机无法接收到。所以甚低频无线电接收甚低频无线电波并将其转换为我们能听到的声音。

嗯，在地球上，自然产生的甚低频辐射的主要来源是闪电，每次闪电都会产生无线电波脉冲&nbsp；啊，是吗，劳拉？

女学生：因为雷雨时几乎总是有闪电，所以我们可以经常探测到VLF波，对吗？你只要等到有雷雨…

男教授：啊，你呢？必须等待吗？甚低频接收机非常灵敏，甚低频微波传播距离非常远。所以我们可以收集远处闪电的辐射。所以，实际上，你几乎可以一直听他们说话，因为闪电不断地击中地球，大约每秒100次。即使你所在的地方没有闪电，通过VLF无线电，你也可以听到数千公里外风暴的噼啪声。

然而，一天中的某些时间比其他时间更适合拾取VLF波。例如，白天不如夜间好。此外，我的同事丹尼斯·加拉赫（DennisGallagher）说，在我看来，他是对的，他说听他们讲话的最佳时间是日落或日出前后。这就是当地大气中存在天然波导的时候。

女学生：你说的是波导管吗？

男教授：是的。波导管，通常是指一种设备，如金属导体，用于引导和引导波。但波导也是自然产生的，它们为无线电波在大气层中的传播开辟了一条路径。这些自然波导发生在太阳升起或落下时，这使得日出和日落都是接收VLF发射的好时机。

现在，你可以在甚低频接收器上听到一些不同的声音，因为当雷击时，无线电波在到达接收器之前以不同的方式传播不同的距离。

一些真正有趣的被称为“哨子”哨声来自闪电产生的无线电波，这些无线电波离开地球大气层，进入地球磁层，然后反弹回来。并非所有无线电波都能做到这一点。他们发出的声音…我们称他们为“哨声”，因为他们听起来像是缓慢下降的音调…没有两个哨声是一样的，呃，对我来说，他们是最有趣的。

啊，另一个有趣的声音是“tweek”TWEEK是VLF波穿过波导长距离传播的结果。它们产生啁啾声，因为波的高频部分先于低频部分到达无线电接收器。整个波仍然被认为是非常低的频率，只是波的某些部分的频率比其他部分低。好啊

三、Electromagnetic Waves托福听力问题：

Q1:1.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

Q2:2.What is one difference between radio waves and sound waves that the professor emphasizes?

A.Radio waves have a lower frequency.

B.Water stops radio waves from spreading but does not stop sound waves

C.Unlike sound waves,radio waves can travel outside Earth's atmosphere.

D.Naturally occurring radio waves are difficult to detect on Earth at night.

Q3:3.What explanation does the professor give for the constant occurrence of VLF emissions on Earth?

A.At any given time,some part of the world is experiencing sunrise or sunset.

B.Waveguides constantly form in the atmosphere.

C.Earth's magnetosphere directs interplanetary waves toward Earth's surface.

D.Lightning occurs constantly on the planet.

Q4:4.Why are sunrise and sunset the best times to listen to VLF signals?

A.Because thunderstorms are most likely to occur then

B.Because radio waves travel through natural waveguides then

C.Because higher-frequency signals are less active then

D.Because temperatures are not extremely high or low then

Q5:5.Why does the professor discuss whistlers and tweeks?

A.To illustrate that the path a VLF wave travels can affect the sound it makes on a radio

B.To point out that VLF waves can affect the sounds heard on a household or car radio

C.To describe how a colleague discovered the origin of VLF waves

D.To clarify the difference between VLF waves and other kinds of waves

Q6:6.Replay:What does the professor imply when he says this:

A.He needs to think before he can answer the woman's question.

B.The woman has underestimated how often VLF waves can be detected.

C.The woman does not realize that waiting for a thunderstorm can take a long time.

D.The woman does not understand the relationship between thunderstorms and lightning.

四、Electromagnetic Waves托福听力答案：

A1:正确答案：D

A2:正确答案：C

A3:正确答案：D

A4:正确答案：B

A5:正确答案：A

A6:正确答案：B

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