Electromagnetic radiation and Radio 电磁波/电磁辐射和无线电波

电磁辐射，又称电磁波，是由同相振荡且互相垂直的电场与磁场在空间中以波的形式传递能量和动量，其传播方向垂直于电场与磁场构成的平面。

电磁辐射的载体为光子，不需要依靠介质传播，在真空中的传播速度为光速。电磁辐射可按照频率分类，从低频率到高频率，主要包括无线电波、微波、红外线、可见光、紫外线、X射线和伽马射线。

人眼可接收到的电磁辐射，波长大约在380至780nm之间，称为可见光。只要是本身温度大于绝对零度的物体，除了暗物质以外，都可以发射电磁辐射，而世界上并不存在温度等于或低于绝对零度的物体，因此，人们周边所有的物体时刻都在进行电磁辐射。尽管如此，只有处于可见光频域以内的电磁波，才可以被人们肉眼看到，对于不同的生物，各种电磁波频段的感知能力也有所不同。

电磁波的频率划分

γ	γ = Gamma rays
X-rays	HX = Hard X-rays	SX = Soft X-Rays
Ultraviolet	EUV = Extreme-ultraviolet	NUV = Near-ultraviolet
Visible light (colored bands)
Infrared	NIR = Near-infrared	MIR = Mid-infrared	FIR = Far-infrared
Microwaves	EHF = Extremely high frequency (microwaves)	SHF = Super-high frequency (microwaves)
Radio	UHF = Ultrahigh frequency (radio waves)	VHF = Very high frequency (radio)	HF = High frequency (radio)	MF = Medium frequency (radio)	LF = Low frequency (radio)

电磁波被大气层吸收的情况

无线电与电磁波
频率在数百千赫到数百兆赫之间的电磁波叫做无线电波，它包括短波、中波、长波、微波，无线电波也仅仅是电磁波的一部分，但电磁波不仅仅只有无线电波，仅仅只有频率相对较低的一部分电磁波才叫无线电波。

波段	频段名称	缩写	频率范围	波段	波长范围	用法
			≤3Hz）		≥ 100,000 Km
1	极低频	ELF	3Hz–30Hz	极长波	100,000千米 – 10,000千米	潜艇通讯或直接转换成声音
2	超低频	SLF	30Hz–300Hz	超长波	10,000千米 – 1,000千米	直接转换成声音或交流输电系统（50-60赫兹）
3	特低频	ULF	300Hz–3KHz	特长波	1,000千米 – 100千米	矿场通讯或直接转换成声音
4	甚低频	VLF	3KHz–30KHz	甚长波	100千米 – 10千米	直接转换成声音、超声、地球物理学研究
5	低频	LF	30KHz–300KHz	长波	10千米 – 1千米	国际广播、全向信标
6	中频	MF	300KHz–3MHz	中波	1千米 – 100米	调幅(AM)广播、全向信标、海事及航空通讯
7	高频	HF	3MHz–30MHz	短波	100米 – 10米	短波、民用电台
8	甚高频	VHF	30MHz–300MHz	米波	10米 – 1米	调频(FM)广播、电视广播、航空通讯
9	特高频	UHF	300MHz–3GHz	分米波	1米 – 100毫米	电视广播、无线电话通讯、无线网络、微波炉
10	超高频	SHF	3GHz–30GHz	厘米波	100毫米 – 10毫米	无线网络、雷达、人造卫星接收
11	极高频	EHF	30GHz–300GHz	毫米波	10毫米 – 1毫米	射电天文学、遥感、人体扫描安检仪
			>300GHz		< 1毫米

电与电磁学的发展

1600, English scientist William Gilbert made a careful study of electricity and magnetism, distinguishing the lodestone effect from static electricity produced by rubbing amber.
1752, Benjamin Franklin was reputed to have attached a metal key to the bottom of a dampened kite string and flown the kite in a storm-threatened sky, a succession of sparks jumping from the key to the back of his hand showed that lightning was indeed electrical in nature.
1800, Alessandro Volta's battery, or voltaic pile, made from alternating layers of zinc and copper, provided scientists with a more reliable source of electrical energy than the electrostatic machines previously used
1819–1820, The recognition of electromagnetism, the unity of electric and magnetic phenomena, is due to Hans Christian Ørsted and André-Marie Ampère.
1821, Michael Faraday invented the electric motor.
1827, Georg Ohm mathematically analysed the electrical circuit.
1837, Morse developed an early forerunner to the modern International Morse code.
1861-1862, Electricity and magnetism (and light) were definitively linked by James Clerk Maxwell, in particular in his "On Physical Lines of Force".
1867, Radio waves were first predicted by mathematical work done by Scottish mathematical physicist James Clerk Maxwell.
1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced clearly intelligible replication of the human voice, which is now called telephone.
1878, in the US, Thomas Edison developed and sold a commercially viable replacement for gas lighting and heating using locally generated and distributed direct current electricity.
1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory, showing that they exhibited the same wave properties as light: standing waves, refraction, diffraction, and polarization
1887, Tesla developed an induction motor that ran on alternating current (AC), a power system format that was rapidly expanding in Europe and the United States because of its advantages in long-distance, high-voltage transmission
1897, Radio waves were first used for communication Guglielmo Marconi, who developed the first practical radio transmitters and receivers.
1904, John Ambrose Fleming invented the two-electrode vacuum-tube rectifier, which he called the oscillation valve. It was also called a thermionic valve, vacuum diode, kenotron, thermionic tube, or Fleming valve. This invention is often considered to have been the beginning of electronics, for this was the first vacuum tube. Fleming's diode was used in radio receivers and radars for many decades afterwards, until it was superseded by solid state electronic technology more than 50 years later.
1906, Lee de Forest invented the first successful three-element (triode) vacuum tube, and the first device which could amplify electrical signals.
1921, The first successful transatlantic tests operating in the 200 meter mediumwave band (near 1,500 kHz in the modern AM broadcast band) – the shortest wavelength then available to amateurs.
1947, The first practically implemented point-contact transistor invented by American physicists John Bardeen, Walter Brattain, and William Shockley.