Section outline

  • General

    Collapse all Expand all

  • 电磁频谱

    What Is The Electromagnetic Spectrum?

    Imagine playing beach volleyball, like the young men in Figure . They may not realize it, but they are being bombarded by electromagnetic radiation as play in the sunlight. The only kinds of radiation they can detect are visible light, which allows them to see, and infrared light, which they feel as warmth on their skin. What other kinds of electromagnetic radiation are they being exposed to in sunlight? In this lesson, you’ll find out.
    ::想象一下海滩排球游戏,就像图中的年轻人一样。他们可能没有意识到这一点,但他们被电磁辐射轰炸,就像在阳光下玩耍一样。他们能够探测到的唯一类型的辐射是可见光,可以让他们看到,还有红外线光,他们感到皮肤是温暖的。在阳光下,他们还受到什么其他类型的电磁辐射?在这个教训中,你会发现。

    Volleyball players engaging in a game on a sunny beach with ocean in background.
    Electromagnetic radiation from the sun reaches Earth across space. It strikes everything on Earth’s surface, including these volleyball players.

    Electromagnetic radiation occurs in waves of different and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum . The electromagnetic spectrum is summarized in the diagram in Figure  .
    ::电磁辐射发生在不同频率的波浪中,红外线光和可见光只是电磁辐射全程的一小部分,即电磁频谱。电磁频谱在图图中作了概括。

    • On the far left of the diagram are , which include . They have the longest wavelengths and lowest frequencies of all . They also have the least amount of .
      ::在图的远左方是, 包括 。 它们有最长的波长和最小的频率 。 它们也有最小的 。
    • On the far right are X rays and . The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy.
      ::最右侧是X光和。所有电磁波的波长最短,频率最高。它们也拥有最大的能量。
    • Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light.
      ::在这两个极端之间,波长、频率和能量从频谱的一面不断变化。电磁频谱中间部分的波浪通常被称为光。

    As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used.
    ::如下文所述,电磁波的特性影响着不同波的运行方式和如何使用。

    The Electromagnetic Spectrum, Showing Wavelengths, Frequencies, And Examples Of Size.
    How do the wavelength and frequency of waves change across the electromagnetic spectrum?

    Speed, Frequency and Wavelength of Electromagnetic Waves
    ::电磁波的速度、频率和波长

    The of an electromagnetic wave depends on the medium it travels through. In a vacuum, all electromagnetic waves, including visible light, travel at the same speed, denoted as "c" and approximately equal to 299,792,458 meters per second.
    ::在真空中,所有电磁波,包括可见光,以同样的速度,以“c”表示,大约等于每秒299,792,458米。

    The speed of a wave can be calculated using the equation, which is expressed as:
    ::海浪的速度可以用以下列公式表示的方程式计算:

    Wave speed = frequency × wavelength
    ::波浪速度 = 频率x波长

    As electromagnetic waves in a vacuum always travel at the same speed, the wave speed equation shows that their frequency and wavelength are inversely proportional. This relationship is expressed by the equation:
    ::由于真空中的电磁波总是以相同速度移动,波速方程式显示其频率和波长反比。

    f = c / λ, or λ = c / f
    ::f = c / / 或 = c / f

    Where "c" is the in a vacuum, "f" is the frequency of the electromagnetic wave, and "λ" is its wavelength.
    ::在“c”是真空中,“f”是电磁波的频率,“”是其波长。

    Radio Waves

    Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies. In Figure  , you can see that the wavelength of radio waves may be longer than a soccer field. With their low frequencies, radio waves have the least energy of electromagnetic waves, but they still are extremely useful. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar. You can learn more about radio waves, including how they were discovered, at this URL: (3:58).
    ::无线电波是具有最长波长和最低频率的电磁波的广度。 在图中,你可以看到无线电波的波长可能比足球场长。 无线电波的频率低,其电磁波的能量最小, 但仍然非常有用。 它们用于无线电和电视广播、 微波炉、 手机传输和雷达。 您可以在这个 URL 上更多地了解无线电波, 包括如何发现这些波, (3: 58) 。

    AM and FM Radio
    ::调调电台和调频电台

    In radio broadcasts, sounds are encoded in radio waves that are sent out through the atmosphere from a radio tower. A receiver detects the radio waves and changes them back to sounds. You’ve probably listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts.
    ::在电台广播中,声音被编码成无线电波,无线电波是从无线电塔通过大气层发出的。接收器检测无线电波,并将其转换为声音。你可能已经收听了调频电台和调频电台。无线电波中声音的编码方式与调频和调频广播不同。

    • AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitude of radio waves. AM broadcasts use longer–wavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM radio waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in Figure  . This allows AM radio waves to reach radio receivers that are very far away from the radio tower.
      ::AM 代表着振幅调控。 在AM 广播中,音频信号通过改变无线电波的振幅来编码。 AM 广播使用比调频广播更长的波长无线电波。 由于波长较长,AM 无线电波从高层大气层中反射出来,称为电离层。您可以看到图中是如何发生的。这使AM 无线电波能够到达离无线电塔很远的无线电接收器。
    • FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually sound clearer than AM broadcasts. However, because of their shorter wavelength, FM waves do not reflect off the ionosphere. Instead, they pass right through it and out into space (see Figure  ). As a result, FM waves cannot reach very distant receivers.
      ::在调频广播中,调频信号通过改变无线电波的频率来编码。调频调频使调频波能够比振幅调控来编码更多的信息,因此调频广播通常比调频广播更清晰。然而,由于调频波的波长较短,调频波不会反射电离层,相反,调频波直接穿过电离层进入空间(见图 )。因此,调频波无法到达非常遥远的接收器。
    Diagram comparing AM and FM radio wave propagation relative to the ionosphere.
    AM radio waves reflect off the ionosphere and travel back to Earth. Radio waves used for FM radio and television pass through the ionosphere and do not reflect back.

    Television
    ::电视电视电视电视电视

    Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one in Figure  . When the waves are received by television sets, they are decoded and changed back to sounds and pictures.
    ::电视广播也使用无线电波,声音用频率调控编码,图片用振幅调控编码,无线电波由像图图中的电视塔那样的电视塔广播。当波浪被电视机接收时,它们就被解码并改成声音和图片。

    Television tower broadcasting signals in a cityscape with buildings and clear sky.
    This television tower broadcasts signals using radio waves.

    Microwaves
    ::微波微波

    The shortest wavelength, highest frequency radio waves are called microwaves (see Figure  ). Microwaves have more energy than other radio waves. That’s why they are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar , which is a device for determining the presence and location of an object by measuring the time for the echo of a radio wave to return from it and the direction from which it returns. These uses are described in Figure  . You can learn more about microwaves and their uses in the video at this URL: (3:23).
    ::最短波长、频率最高的无线电波称为微波(见图)。微波比其他无线电波的能量要大。这就是为什么微波对微波炉的供暖食品有用。微波还有其他重要用途,包括手机传输和雷达,这是通过测量无线电波回声从中返回的时间和返回的方向来确定物体的存在和位置的装置。这些用途在图中描述。你可以在这个URL的视频中更多地了解微波及其用途sad3:23)。

    Microwaves Used In Cell Phones And Radar Systems.
    Microwaves are used for cell phones and radar.

    Light

    Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorter wavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Light includes visible light, infrared light, and ultraviolet light. If you look back at Figure , you can see where these different types of light waves fall in the electromagnetic spectrum.
    ::中等波长电磁波通常称为光。 这种电磁波的波长较短,频率也比无线电波高,但没有X射线和伽马射线那么短和高。光包括可见光、红外光和紫外光。如果你回顾图,你可以看到电磁波中不同种类的光波落到哪里。

    Visible Light
    ::可见光灯

    The only light that people can see is called visible light . It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure  . When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter "Visible Light" and at the URL below.
    ::人们唯一能看到的光线被称为可见光。 它指红外光和紫外线光之间电磁波谱中非常狭窄的波长范围。 在可见范围内, 我们将不同波长的光作为不同的光的颜色来看待, 从红光(有最长波长)到紫外光(有最短波长), 从紫外光(有最短波长)到紫外光(有最短波长)。 您可以在图中看到可见光的频谱。 当所有波长(如在阳光下的波长)被组合在一起时, 可见光会变得白色。 您可以在“ 可见光” 章和下面的 URL 中更多地了解可见光。

    (4:50)

    A spectrum displaying colors from violet on the left to red on the right.
    Red light (right) has the longest wavelength, and violet light (left) has the shortest wavelength.

    Infrared Light
    ::红外光

    Light with the longest wavelengths is called infrared light . The term infrared means "below red." Infrared light is the range of light waves that have longer wavelengths than red light in the visible spectrum. You can’t see infrared light waves, but you can feel them as on your skin. The sun gives off infrared light as do fires and living things. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves and changes their energy to colored light in the visible range. Night vision goggles, which are used by law enforcement and the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For a deeper understanding of infrared light, watch the video at this URL: (6:46).
    ::有最长波长的光被称作红外线光。红外线是指“红下”。红外光指在可见光谱中波长比红光长的光波范围。您无法看到红外光波,但能感觉到它们像皮肤上的光波。太阳像火和活物一样释放红外光光。打开本章的一只猫的图片是用照相机制作的,摄像头可以探测红外光波,并将它们的能量改变为可见光的彩色光。执法人员和军方使用的夜视镜也能够探测红外光波。这些护目镜将无形波转换成可见的图像。为了更深入地了解红外光和活物,请看这个URL的视频: (6: 46) 。

    Ultraviolet Light
    ::紫外光

    Light with wavelengths shorter than visible light is called ultraviolet light . The term ultraviolet means "above violet." Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visible spectrum. Humans can’t see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves than visible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (see Figure  ). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL: (3:40).
    ::紫外线是指“紫外线以上”。紫外线光是光波的范围,其波长比可见光谱中的紫外线光短。人类看不到紫外线光,但它非常有用。它有比可见光高频波,因此它有更大的能量。它可以用来杀死食物中的细菌和对实验室设备进行消毒(见图 )。人类皮肤在接触紫外线光时,也使维生素D成为维生素D。坚固的骨骼和牙齿需要维生素D。你可以更多地了解紫外线光及其在这个URL上的发现sad3:40)。

    A laboratory sterilizer using ultraviolet light, intended for sterilizing equipment effectively.
    This sterilizer for laboratory equipment uses ultraviolet light to kill bacteria.

    Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultraviolet light by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun-protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure ). A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 even on cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often. You can learn more about the effects of ultraviolet light on the skin at this URL:  (4:45).
    ::太多的紫外线光照射会导致太阳晒伤和皮肤癌。 您可以通过穿戴覆盖皮肤的衣物和在任何暴露地区使用防晒霜来保护自己的皮肤不受紫外线光照射。 防晒霜的防晒系数(SPF)粗略地说明了它保护皮肤免受太阳灼伤的时间(见图 ) 。 防晒霜加上更高的防晒霜可以保护皮肤更长的时间。 您应该用防晒霜来保护皮肤。 因为紫外线光可以在云中穿透。 日光应宽度和频繁地应用。 您可以在此URL( 4: 45) 了解更多紫外线光对皮肤的影响 。

    Close-up of a sunscreen bottle showing SPF 30 label.
    If your skin normally burns in 10 minutes of sun exposure, using sunscreen with an SPF of 30 means that, ideally, your skin will burn only after 30 times 10 minutes, or 300 minutes, of sun exposure. How long does sunscreen with an SPF of 50 protect skin from sunburn?

    X Rays and Gamma Rays

    The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays have so much energy that they can pass through many materials. This makes them potentially very harmful, but it also makes them useful for certain purposes.
    ::最短波长、频率最高的电磁波是X射线和伽马射线,这些射线的能量巨大,能够通过许多材料。这有可能导致非常有害,但也使这些射线在某些用途上有用。

    X Rays
    ::X射线

    X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skin but not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figure  show where X rays were absorbed by the teeth. X rays are used not only for dental and medical purposes but also to screen luggage at airports (see Figure  ). Too much X ray exposure may cause cancer. If you’ve had dental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays. The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as well as other uses of X rays at this URL: .
    ::X光是高能电磁波,它们有足够的能量通过皮肤等软组织,但不足以通过非常稠密的骨头和牙齿。图中X光片中的亮点显示牙齿吸收X光的地方。X光不仅用于牙科和医疗目的,而且用于机场的检查行李(见图 )。太多X光照射可能导致癌症。如果你有牙科X光,你可能已经注意到在你的身上放置了一个重围裙,以保护它不受偏离的X光线的影响。围裙是由铅制成的,X光线无法通过。你可以知道在这个URL发现了X光以及X光的其他用途:

    Dental X ray showing teeth; Airport X ray screening luggage for security.
    Two common uses of X rays are illustrated here.

    Gamma Rays
    ::伽马射线

    Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, including bones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medical device sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn more about gamma rays, watch the video at the URL below.
    ::伽马射线是所有电磁波中最活跃的射线。 它们可以通过大多数材料, 包括骨头和牙齿。 尽管如此, 这些波浪还是有用的。 例如, 它们可以用来治疗癌症。 医疗设备将伽马射线发送到癌症现场, 射线会摧毁癌细胞。 如果您想了解更多伽马射线, 请在下面的 URL 上观看视频 。

    (2:45)

    KQED: Seeing Cells in 3-D
    ::KQED: 3D 显示单元格

    Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in a manner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping to advance research into the development of biofuels, the treatment of malaria and it may even help to more rapidly diagnose cancer. For more information on X ray microscopes, see .
    ::伯克利的科学家开发了一个强大的新显微镜,它利用X光扫描整个细胞,以几分钟的方式,对细胞及其遗传物质产生三维视图。 这一开创性工具有助于推进对生物燃料开发、疟疾治疗的研究,甚至有助于更快地诊断癌症。 关于X光显微镜的更多信息,请参见 。

    Launch the Light Wave simulation below to visualize the different types of light waves. Be sure to check out the objects of comparable size next to the Electromagnetic Spectrum to help you grasp just how big or small different waves of light can be:
    ::启动下面的光波模拟, 以视觉化不同种类的光波。 一定要检查电子磁共振光谱旁边大小相似的天体, 以便帮助您了解不同光波的大小 :