📚 Electromagnetic Waves | 电磁波
Electromagnetic waves are a central topic in CIE A-Level Physics. They transfer energy without requiring a medium, and they include a continuous spectrum from radio waves to gamma rays. Understanding their common properties, the wave equation, polarisation, photon energy, production methods and applications is essential for both AS and A2 papers.
电磁波是 CIE A-Level 物理的核心主题之一。它们不需要介质即可传递能量，并构成从无线电波到伽马射线的连续谱。掌握它们的共同性质、波动方程、偏振、光子能量、产生方法以及应用，对 AS 和 A2 考试都至关重要。
1. What Are Electromagnetic Waves? | 什么是电磁波？
Electromagnetic (EM) waves are transverse waves produced by accelerating charges. An EM wave consists of oscillating electric and magnetic fields. These fields are perpendicular to each other and to the direction of wave travel, and they regenerate each other as the wave moves through space.
电磁波是由加速电荷产生的横波。电磁波由振荡的电场和磁场组成。这两个场相互垂直，并与波的传播方向垂直；当波在空间中传播时，电场和磁场相互再生。
A key point for CIE exams is that EM waves do not need a material medium. They can travel through a vacuum at the speed of light, carrying energy and momentum from one place to another.
CIE 考试的一个关键点是：电磁波不需要物质介质。它们可以在真空中以光速传播，并把能量和动量从一个地方传递到另一个地方。
2. Common Properties and the Wave Equation | 共同性质与波动方程
In a vacuum, all electromagnetic waves travel at the same speed c, where c = 3.00 × 10⁸ m s⁻¹. The relationship between speed, frequency and wavelength is:
在真空中，所有电磁波以相同的速度 c 传播，c = 3.00 × 10⁸ m s⁻¹。速度、频率和波长之间的关系为：
c = fλ
Here c is the speed in metres per second, f is the frequency in hertz, and λ is the wavelength in metres. Because c is constant in a vacuum, a large frequency must correspond to a small wavelength.
其中 c 以米每秒为单位，f 以赫兹为单位，λ 以米为单位。由于 c 在真空中恒定，频率大对应的波长一定小。
Worked example: a microwave has frequency f = 2.45 × 10⁹ Hz. Its wavelength in vacuum is λ = c / f = 3.00 × 10⁸ / 2.45 × 10⁹ ≈ 0.122 m.
例题：一束微波的频率 f = 2.45 × 10⁹ Hz。它在真空中的波长为 λ = c / f = 3.00 × 10⁸ / 2.45 × 10⁹ ≈ 0.122 m。
3. Transverse Nature and Polarisation | 横波特性与偏振
Electromagnetic waves are transverse: the electric and magnetic field oscillations are perpendicular to the direction of energy transfer. This transverse nature can be demonstrated by polarisation.
电磁波是横波：电场和磁场的振荡方向与能量传递方向垂直。这种横波特性可以通过偏振来证明。
Polarisation is the process of restricting the oscillations of a transverse wave to one plane. A polarising filter only transmits the component of the electric field that is parallel to its transmission axis. This can be observed with visible light and two polaroid filters; rotating one filter changes the transmitted intensity.
偏振是将横波的振荡限制在一个平面内的过程。偏振片只让平行于其透振轴的电场分量通过。用可见光和两个偏振片可以观察到这一现象；转动其中一个偏振片会改变透射光的强度。
Because only transverse waves can be polarised, polarisation provides evidence that electromagnetic waves are transverse, not longitudinal. Microwaves can also be polarised using a metal grid with parallel wires.
由于只有横波才能被偏振，偏振为电磁波是横波而非纵波提供了证据。微波也可以用平行金属线栅来实现偏振。
4. Ordering the Electromagnetic Spectrum | 电磁波谱的排列
The electromagnetic spectrum is a continuous range of wavelengths and frequencies. From longest wavelength (lowest frequency, lowest photon energy) to shortest wavelength (highest frequency, highest photon energy), the main regions are:
电磁波谱是一个连续的波长和频率范围。从长波长（低频、低光子能量）到短波长（高频、高光子能量），主要区域依次为：
| Radiation 波段 | Approximate wavelength 近似波长 | Relative frequency and energy 相对频率与能量 |
|---|---|---|
| Radio waves 无线电波 | > 10⁻¹ m | Lowest 最低 |
| Microwaves 微波 | 10⁻¹ m to 10⁻³ m | Low 低 |
| Infrared 红外线 | 10⁻³ m to 7 × 10⁻⁷ m | Moderate 中等 |
| Visible light 可见光 | 7 × 10⁻⁷ m to 4 × 10⁻⁷ m | Medium 中 |
| Ultraviolet 紫外线 | 4 × 10⁻⁷ m to 10⁻⁸ m | High 高 |
| X-rays X 射线 | 10⁻⁸ m to 10⁻¹³ m | Very high 很高 |
| Gamma rays 伽马射线 | < 10⁻¹¹ m | Highest 最高 |
Remember that the boundaries between regions are not sharp; they overlap slightly depending on the source and the convention used. In CIE questions, you are usually expected to know the order and the relative wavelength, frequency and photon energy.
请记住，各波段之间的边界并不是严格的；根据来源和约定不同，它们会略有重叠。在 CIE 考题中，通常要求掌握顺序以及波长、频率和光子能量的相对大小。
5. Radio Waves and Microwaves | 无线电波与微波
Radio waves are produced by alternating currents in transmitting aerials. They are detected by receiving aerials in which the electric field of the wave drives electrons to create a small alternating current. Typical uses include radio and television broadcasting, as well as long-distance communication.
无线电波由发射天线中的交变电流产生。它们由接收天线检测：波中的电场驱动电子，形成微小的交变电流。典型用途包括无线电和电视广播以及长距离通信。
Microwaves are produced by magnetron or klystron devices and detected by microwave diodes. They are used in radar, satellite communication, Wi-Fi and microwave ovens. In a microwave oven, microwaves cause polar water molecules in food to rotate rapidly, so their kinetic energy increases and the food heats up internally.
微波由磁控管或速调管产生，并由微波二极管检测。它们用于雷达、卫星通信、Wi-Fi 和微波炉。在微波炉中，微波使食物中的极性水分子快速旋转，使其动能增大，从而使食物从内部被加热。
A hazard of microwaves is internal heating of body tissue, especially in organs such as the eyes where blood flow is limited and heat is not easily removed.
微波的危害之一是对人体组织的内部加热，尤其是眼睛等血流有限、热量不易散失的器官。
6. Infrared, Visible and Ultraviolet | 红外线、可见光与紫外线
Infrared radiation is emitted by hot objects and by molecular vibrations. It can be detected with a thermopile, thermistor or infrared photodiode. Applications include thermal imaging, remote controls, infrared heaters and fibre-optic communication.
红外线由高温物体和分子振动发出。它可以用热电堆、热敏电阻或红外光电二极管检测。应用包括热成像、遥控器、红外加热器和光纤通信。
Visible light is produced when electrons in atoms fall from higher to lower energy levels. It is detected by the human eye, charge-coupled devices (CCDs) and photographic film. Visible light is used for vision, photography and optical fibre communication.
可见光是原子中的电子从高能级跃迁到低能级时产生的。它由人眼、电荷耦合器件（CCD）和照相胶片检测。可见光用于视觉、摄影和光纤通信。
Ultraviolet radiation has higher photon energy than visible light. It is produced by high-energy atomic electron transitions, mercury vapour lamps and the Sun. It can be detected using fluorescent materials or photodiodes. UV is used for sterilising surfaces, detecting forged banknotes and tanning. However, overexposure can cause sunburn and skin cancer.
紫外线的光子能量高于可见光。它由高能原子电子跃迁、汞蒸气灯和太阳产生。可以用荧光材料或光电二极管检测。紫外线用于表面消毒、检测伪钞和晒黑皮肤。但过度暴露会导致晒伤和皮肤癌。
7. X-rays and Gamma Rays | X 射线与伽马射线
X-rays are produced when high-speed electrons strike a metal target, or when inner-shell electrons undergo transitions in atoms. They are detected by photographic film, Geiger counters or ionisation chambers. X-rays are used in medical imaging, airport security scanners and X-ray crystallography. They are ionising and can damage living cells, so exposure must be kept as low as possible.
X 射线由高速电子撞击金属靶或原子内层电子发生跃迁时产生。它们由照相胶片、盖革计数器或电离室检测。X 射线用于医学成像、机场安检扫描和 X 射线晶体学。它们具有电离能力，会损伤活细胞，因此应尽量减少暴露。
Gamma rays are produced by radioactive decay in atomic nuclei. They are detected by Geiger-Müller tubes and scintillation counters. Gamma rays are used in cancer radiotherapy, sterilising medical equipment and industrial radiography. They are the most penetrating and most ionising part of the electromagnetic spectrum, so they require thick lead or concrete shielding.
伽马射线由原子核的放射性衰变产生。它们由盖革-米勒计数管和闪烁计数器检测。伽马射线用于癌症放射治疗、医疗设备消毒和工业射线照相。它们是电磁波谱中穿透力最强、电离能力最强的部分，因此需要用厚铅板或混凝土进行屏蔽。
In exams, distinguish X-rays and gamma rays by their origin rather than by energy: X-rays come from electron interactions, while gamma rays come from nuclear changes.
在考试中，应依据来源而非能量来区分 X 射线和伽马射线：X 射线来自电子相互作用，而伽马射线来自原子核变化。
8. Energy, Frequency and the Photon Model | 能量、频率与光子模型
Electromagnetic radiation can behave as a stream of photons. The energy of each photon is directly proportional to the frequency of the radiation:
电磁辐射可以表现为光子流。每个光子的能量与辐射频率成正比：
E = hf
Here h is the Planck constant, h = 6.63 × 10⁻³⁴ J s. Since c = fλ, the photon energy can also be written as E = hc / λ. A shorter wavelength therefore means a higher photon energy.
其中 h 是普朗克常数，h = 6.63 × 10⁻³⁴ J s。由于 c = fλ，光子能量也可以写成 E = hc / λ。因此波长越短，光子能量越高。
Worked example: red visible light has a frequency of about
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