18.2 电磁波的属性

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  电磁波属性

  

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  What do these two photos have in common? They both represent . These are waves that consist of vibrating electric and magnetic fields. They transmit through matter or across space. Some electromagnetic waves are generally harmless. The light we use to see is a good example. Other electromagnetic waves can be very harmful and care must be taken to avoid too much exposure to them. X rays are a familiar example. Why do electromagnetic waves vary in these ways? It depends on their properties. Like other waves, electromagnetic waves have properties of , , and frequency.
  ::这两个照片有什么共同之处? 它们都代表着什么? 它们都是由振动电磁场和电磁场组成的波。 它们通过物质或空间传播。 一些电磁波一般无害。 我们用来看到的光是一个很好的例子。 其他电磁波可能非常有害, 并且必须小心避免过多地接触它们。 X 射线是一个熟悉的例子。 为什么电磁波在这些方面变化? 它取决于它们的特性。 和其他波一样, 电磁波有其特性, 以及频率。

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  Speed of Electromagnetic Waves
  ::电磁波速度

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  All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light , is about 300 million meters per second (3.0 x 10 ⁸ m/s). Nothing else in the universe is known to travel this fast. The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! 
  ::所有电磁波都以同样的速度在空空空空间中运行。 光速被称为光速, 大约每秒3亿米( 3. 0 x 108 m/s) 。 宇宙中没有任何其它东西可以如此快速运行。 太阳距离地球约1.5亿公里( 9 300万英里) , 但是从太阳到达地球只需要8分钟电磁辐射。 如果您能够如此快速移动, 您就可以在1秒内绕地球行7. 5 次 !

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  Wavelength and Frequency of Electromagnetic Waves
  ::电磁波波的波长和频率

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  Although all electromagnetic waves travel at the same speed across space, they may differ in their wavelengths, frequencies, and energy levels.
  ::尽管所有电磁波以同样的速度在空间之间移动,但它们的波长、频率和能量水平可能有所不同。

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  • Wavelength is the between corresponding points of adjacent waves (see the Figure  ). Wavelengths of electromagnetic waves range from longer than a soccer field to shorter than the diameter of an .
    ::波长是相邻波的对应点之间的波长(见图)。电磁波的波长范围从足球场长到直径短不等。
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  • is the number of waves that pass a fixed point in a given amount of time. Frequencies of electromagnetic waves range from thousands of waves per second to trillions of waves per second.
    ::是在一定时间内通过固定点的波数。电磁波的频率从每秒数千波到每秒数万亿波不等。
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  • The energy of electromagnetic waves depends on their frequency. Low-frequency waves have little energy and are normally harmless. High-frequency waves have a lot of energy and are potentially very harmful.
    ::电磁波的能量取决于它们的频率,低频波的能量很少,而且通常无害。高频波的能量很多,而且可能非常有害。

  

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  Q: Which electromagnetic waves do you think have higher frequencies: visible light or X rays?
  ::问题:你认为哪个电磁波的频率较高:可见光或X光?

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  A: X rays are harmful but visible light is harmless, so you can infer that X rays have higher frequencies than visible light.
  ::A:X光是有害的,但可见光是无害的,因此可以推断X光的频率高于可见光。

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  Speed, Wavelength, and Frequency
  ::速度、波长和频率

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  The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation:
  ::海浪的速度是波长和频率的产物。由于所有电磁波都以同样的速度在空间中移动,波长较短的波必须具有更高的频率,反之亦然。

  Speed = Wavelength × Frequency

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  The equation for can be rewritten as:
  ::方程式可改写为:

  $\begin{align*}\text{Frequency} = \frac{\text{Speed}}{\text{Wavelength}}\end{align*}$ or $\begin{align*}\text{Wavelength} = \frac{\text{Speed}}{\text{Frequency}}\end{align*}$

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  Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromagnetic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0 × 10 ⁸ meters per second. Its frequency can be found by substituting these values into the frequency equation:
  ::因此, 如果已知波长或频率, 可以计算缺失值 。 考虑一个波长为 3 米的电磁波 。 其速度, 和所有电磁波的速度一样, 每秒3. 0 × 108 米。 其频率可以通过将这些值替换为频率方程式来找到 :

  $\begin{align*}\text{Frequency} = \frac{3.0 \times 10^8 \ \text{m/s}}{3.0 \ \text{m}} =1 .0 \times 10^8 \ \text{waves/s}\end{align*}$ , or 1.0 × 10 ⁸ Hz

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  Q: What is the wavelength of an electromagnetic wave that has a frequency of 3.0 × 10 ⁸ hertz ?
  ::问题:频率为3.0×108赫兹的电磁波的波长是多少?

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  A: Use the wavelength equation:
  ::A:使用波长方程式:

  $\begin{align*}\text{Wavelength} = \frac{3.0 \times 10^8 \ \text{m/s}}{3.0 \times 10^8 \ \text{waves/s}}=1.0 \ \text{m}\end{align*}$

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  Summary
  ::摘要

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  • All electromagnetic waves travel across space at the speed of light, which is about 300 million meters per second (3.0 x 10 ⁸ m/s).
    ::所有电磁波均以光速穿越空间,每秒约3亿米(3.0x108米/秒)。
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  • Electromagnetic waves vary in wavelength and frequency. Longer wavelength electromagnetic waves have lower frequencies, and shorter wavelength waves have higher frequencies. Higher frequency waves have more energy.
    ::电磁波的波长和频率各不相同,较长波长电磁波的频率较低,较短波长波的频率较高,高频波的能量较多。
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  • The speed of a wave is a product of its wavelength and frequency. Because the speed of electromagnetic waves through space is constant, the wavelength or frequency of an electromagnetic wave can be calculated if the other value is known.
    ::电波的速度是波长和频率的产物。由于电磁波在空间中的速度是恒定的,所以如果知道另一个值,就可以计算电磁波的波长或频率。

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  Review
  ::回顾

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  1. What is the speed of light across space?
     ::穿越空间的光速是多少?
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  2. Describe the range of wavelengths and frequencies of electromagnetic waves.
     ::描述电磁波波波长和频率的范围。
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  3. How is the energy of an electromagnetic wave related to its frequency?
     ::电磁波的能量如何与其频率相关?
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  4. If the frequency of an electromagnetic wave is 6.0 × 10 ⁸ Hz, what is its wavelength?
     ::如果电磁波的频率是6.0×108赫兹,其波长是多少?