The Nature of Electromagnetic Waves | 电磁波的本质

📚 The Nature of Electromagnetic Waves | 电磁波的本质

Electromagnetic waves are oscillations of electric and magnetic fields that propagate through space, even in a vacuum. They are fundamental to the study of optics, communications, and modern physics. In the CIE A Level syllabus, you need to understand their transverse nature, the electromagnetic spectrum, speed, polarisation, and energy transfer.

电磁波是电场和磁场的振荡，在空间甚至真空中传播。它们是光学、通信和现代物理学研究的基础。在CIE A Level大纲中，你需要理解电磁波的横波本质、电磁波谱、传播速度、偏振和能量传递。

1. Definition and Nature | 定义与本质

An electromagnetic wave consists of an oscillating electric field and an oscillating magnetic field. The two fields are perpendicular to each other and to the direction of wave travel, which makes electromagnetic waves transverse.

电磁波由振荡的电场和振荡的磁场组成。两个场彼此垂直，并且都垂直于波的传播方向，因此电磁波是横波。

Because a changing electric field generates a magnetic field and a changing magnetic field generates an electric field, the wave is self-sustaining and does not require a material medium.

由于变化的电场产生磁场，变化的磁场产生电场，电磁波能够自持传播，不需要介质。

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2. The Electromagnetic Spectrum | 电磁波谱

The electromagnetic spectrum arranges all electromagnetic waves in order of wavelength and frequency. All regions travel at the same speed in a vacuum, but they differ in wavelength and in the way they interact with matter.

电磁波谱按照波长和频率排列所有电磁波。所有波段在真空中传播速度相同，但波长以及与物质相互作用的方式不同。

Band | 波段	Approximate wavelength | 近似波长	Frequency range | 频率范围
Radio waves | 无线电波	> 0.1 m	< 3 × 10⁹ Hz
Microwaves | 微波	0.1 m – 1 mm	3 × 10⁹ – 3 × 10¹¹ Hz
Infrared | 红外线	1 mm – 700 nm	3 × 10¹¹ – 4.3 × 10¹⁴ Hz
Visible light | 可见光	700 nm – 400 nm	4.3 × 10¹⁴ – 7.5 × 10¹⁴ Hz
Ultraviolet | 紫外线	400 nm – 10 nm	7.5 × 10¹⁴ – 3 × 10¹⁶ Hz
X-rays | X射线	10 nm – 0.01 nm	3 × 10¹⁶ – 3 × 10¹⁹ Hz
Gamma rays | 伽马射线	< 0.01 nm	> 3 × 10¹⁹ Hz

The product of frequency and wavelength is always equal to the speed of light in a vacuum, so high-frequency waves have very short wavelengths.

频率和波长的乘积始终等于真空中的光速，因此高频波的波长非常短。

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3. Production and Detection | 产生与探测

Electromagnetic waves are produced whenever charged particles accelerate. An alternating current in a metal antenna produces radio waves; thermal vibrations of atoms and molecules produce infrared; electron transitions between energy levels produce visible and ultraviolet light.

只要带电粒子加速，就会产生电磁波。金属天线中的交流电产生无线电波；原子和分子的热振动产生红外线；电子在能级之间的跃迁产生可见光和紫外线。

Detection methods depend on photon energy and wavelength: radio waves are detected by antennas, microwaves by diodes, infrared by thermopiles, visible light by the eye or CCD, ultraviolet by photomultipliers, X-rays by photographic film or scintillators, and gamma rays by Geiger counters.

探测方法取决于光子能量和波长：无线电波用天线探测，微波用二极管，红外线用热电堆，可见光用眼睛或CCD，紫外线用光电倍增管，X射线用照相胶片或闪烁体，伽马射线用盖革计数器。

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4. Speed of Electromagnetic Waves | 电磁波的速度

All electromagnetic waves travel at the same speed in a vacuum: c = 3.00 × 10⁸ m s⁻¹. This speed does not depend on frequency or wavelength.

所有电磁波在真空中的传播速度相同：c = 3.00 × 10⁸ m s⁻¹。这个速度不依赖于频率或波长。

Maxwell showed that c = 1/√(μ₀ε₀), where μ₀ is the permeability of free space and ε₀ is the permittivity of free space. This result linked optics with electromagnetism and predicted that light is an electromagnetic wave.

麦克斯韦证明 c = 1/√(μ₀ε₀)，其中 μ₀ 是真空磁导率，ε₀ 是真空电容率。这一结果将光学与电磁学联系起来，并预言光是一种电磁波。

c = fλ and c = 1/√(μ₀ε₀)

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5. Transverse Nature and Polarisation | 横波本质与偏振

Only transverse waves can be polarised. When unpolarised light passes through a polarising filter, the transmitted wave oscillates only in the plane of the filter’s transmission axis. This behaviour is direct evidence for the transverse nature of electromagnetic waves.

只有横波才能被偏振。当非偏振光通过偏振片时，透射波只沿偏振片的透振方向振动。这一行为是电磁波横波本质的直接证据。

If plane-polarised light of intensity I₀ is incident on a second polariser at angle θ to the first, the transmitted intensity obeys Malus’s law.

如果强度为 I₀ 的平面偏振光入射到与第一偏振片成 θ 角的第二偏振片上，透射强度遵循马吕斯定律。

I = I₀ cos²θ

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6. Electric and Magnetic Fields in Phase | 电场与磁场的同相关系

In an electromagnetic wave, the electric and magnetic fields reach their maximum and minimum values at the same points along the wave; they are in phase. The fields are perpendicular to each other and to the direction of propagation.

在电磁波中，电场和磁场在波上的同一点同时达到最大值和最小值，它们是同相的。两个场彼此垂直并垂直于传播方向。

The ratio of the electric field strength E to the magnetic field strength B equals the speed of the wave in a vacuum. If one field is known, the other can be calculated.

电场强度 E 与磁场强度 B 之比等于波在真空中的传播速度。如果已知其中一个场，就可以计算另一个场。

E/B = c or E₀ = cB₀

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7. Energy Transport and Intensity | 能量传递与强度

Electromagnetic waves transport energy from one place to another. The energy density in a region of an electromagnetic wave is shared equally between the electric and magnetic fields.

电磁波将能量从一个地方传递到另一个地方。电磁波中某一区域的能量密度由电场和磁场平均分配。

Intensity is defined as power per unit cross-sectional area. For a plane wave, intensity is proportional to the square of the electric field amplitude. For a point source radiating uniformly, the intensity follows an inverse square law.

强度定义为单位横截面积上的功率。对于平面波，强度与电场振幅的平方成正比。对于均匀辐射的点源，强度遵循平方反比定律。

I ∝ E₀² and I = P/(4πr²)

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8. Momentum and Radiation Pressure | 动量与辐射压

Although photons have no rest mass, electromagnetic waves carry momentum. For a photon of energy E, the momentum is p = E/c.

尽管光子没有静止质量，电磁波仍携带动量。对于能量为 E 的光子，其动量为 p = E/c。

When electromagnetic radiation is absorbed by a surface, it exerts a pressure of I/c; when reflected normally, the pressure is 2I/c. This radiation pressure explains why comet tails point away from the Sun and is the principle behind solar sails.

当电磁辐射被表面吸收时，产生 I/c 的辐射压强；当垂直反射时，压强为 2I/c。这种辐射压解释了彗尾背向太阳的原因，也是太阳帆的原理。

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9. Polarisation Applications | 偏振的应用

Polarisation has many practical uses: Polaroid sunglasses block horizontally polarised glare from water and roads; crossed polarisers in stress analysis reveal internal stresses in transparent plastics; radio and TV antennas must be aligned with the polarisation of the transmitted wave for maximum signal.

Published by TutorHao | A-Level Physics Revision Series | aleveler.com

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