1.Listen to part of a lecture in a physics class.
请听物理学课上的部分内容。
2.The professor has been discussing electromagnetic waves.
教授正在讨论电磁波。
3.So are there any questions before we continue our discussion of different types of electromagnetic waves?
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?
4.Um, today, we'll focus on radio waves, and specifically, very low frequency radio waves.
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。
6.Are you going to talk at all about the difference between radio waves and sound waves?
你会讲无线电波和声波之间的区别吗?
8.That might be a good place to start actually.
事实上,这也许是个不错的切入点。
9.Sound waves are mechanical in nature, right?
声波本质上属于力学,对吧?
10.They can only originate and spread in places where there's some dense physical medium, like atmosphere or water.
它只能在有一些密实的物理介质的环境中产生和传播,比如空气和水。
11.They result from changes in pressure in that medium, like changes in air pressure.
它们因那种介质中压力的变化而产生,比如气压的变化。
12.So they can't travel through a vacuum, where there is no dense physical medium, which is why they can't travel through interplanetary space.
所以它们不能在没有密实物理介质的真空环境中传播,这也就是为什么它们无法穿过行星间的太空了。
13.Radio waves, on the other hand, are fundamentally different from sound waves.
而无线电波和声波有着本质的区别。
14.They are electromagnetic.
它们是电磁波。
15.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.
它们从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。
16.Now, radio waves can be detected.
无线电波是可以探测到的。
17.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.
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,它能接收到很长低频次的无线电波,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。
18.So VLF radios pick up VLF radio waves and convert them to sounds we can hear.
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。