The Electromagnetic Spectrum

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

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Discussion

introduction

A good, general sequence to remember is radio waves, microwaves, infrared, light, ultraviolet, x-rays, gamma rays

special-table-spectrum.html

micropulsations
- small, almost sinusoidal fluctuations of the geomagnetic field, usually with durations of seconds to minutes

radio waves

oscillating, electric circuits
discovered in 1888
micropulsations, electric power transmission, analog audio signals, radio transmission, microwaves
ELF, SLF, ULF, VLF, LF, MF, HF, VHF, UHF, SHF, EHF

ITU	frequency			wavelength			name		alternate name
Radio Frequency Bands
1	3	–	30 Hz	100,000	–	10,000 km	extremely low frequency	(ELF)		(ELF1)
2	30	–	300 Hz	10,000	–	1000 km	super low frequency	(SLF)		(ELF2)
3	300	–	3000 Hz	1000	–	100 km	ultra low frequency	(ULF)		(ELF3)
4	3	–	30 kHz	100	–	10 km	very low frequency	(VLF)
5	30	–	300 kHz	10	–	1 km	low frequency	(LF)	long wave	(LW)
6	300	–	3000 kHz	1000	–	100 m	medium frequency	(MF)	medium wave	(MW)
7	3	–	30 MHz	100	–	10 m	high frequency	(HF)	short wave	(SW)
8	30	–	300 MHz	10	–	1 m	very high frequency	(VHF)
9	300	–	3000 MHz	1000	–	100 mm	ultra high frequency	(UHF)	⎫ ⎬ microwaves ⎭
10	3	–	30 GHz	100	–	10 mm	super high frequency	(SHF)
11	30	–	300 GHz	10	–	1 mm	extremely high frequency	(EHF)
12	0.3	–	3 THz	1	–	0.1 mm			far infrared	(FIR)

microwaves

radiation which has a wavelength between 1 mm and 30 cm and frequencies between 1 GHz and 300 GHz
rotating polar molecules
cavity resonator magnetron
cosmic background radiation
radio detection and ranging (RADAR)
microwave oven

Equivalent Circuit Diagram

Diagram from Percy Spencer's 1945 US patent for the microwave oven. Note the popcorn kernel labeled "Food to be Cooked"

radar

The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world in the 21st century. They have been adopted in the United States by the IEEE, and internationally by the ITU.

Military radar band nomenclature (L, S, C, X, Ku, K and Ka bands) originated during World War II as a secret code so scientists and engineers could talk about frequencies without divulging them. After the war the codes were declassified, millimeter (mm) was added, and the designations were eventually adopted by the IEEE -- Institute of Electric and Electronic Engineers. Military radar band nomenclature is widely used today in radar, satellite and terrestrial communications, and electronic countermeasure applications, both military and commercial.

		frequency range (GHz)
Radar Frequency Bands
band		IEEE			ITU			TVRO
L	"long"	1	–	2	01.215	–	01.400
S	"short"	2	–	4	02.300	–	02.500	1.70	–	3.00
C	"compromise"	4	–	8	05.250	–	05.925	3.70	–	4.20
X		8	–	12	08.500	–	10.680
K_u	"kurze under"	12	–	18	13.400	–	14.000	10.90	–	11.75
					15.700	–	17.700	11.75	–	12.50
								12.50	–	12.75
K	"kurze" (german for short)	18	–	27	24.050	–	24.250
K_a	"kurze above"	27	–	40	33.400	–	36.000	18.00	–	20.00
V		40	–	75
W		75	–	110

infrared (a.k.a. "infrared light")
- "below" red
- a pigment of the imagination
- radiation in the wavelength range 0.7 micrometer to 1 millimeter
- discovered in 1800 by William Herschel (1738-1822) in the sun's spectrum
- vibrating molecules
- atoms in solids vibrating about their lattice positions
- Humans usually perceive infrared radiation as heat.
- "not so¯hot" stuff
  - thermal infrared Infrared radiation which has a wavelength between 30 μm and 200 μm. At normal environmental temperatures objects emit infrared between these wavelengths; hotter objects, such as fires, emit infrared at wavelengths shorter than thermal infrared.
  - mid-infrared Infrared radiation which has a wavelength between 5 μm and 30 μm
  - near-infrared Infrared radiation which has a wavelength between 0.7 μm and 5 μm. Near-infrared is further subdivided into
    - short-wave infrared (1 μm-5 μm).
    - very-near infrared (0.7 μm-1 μm) Photographic films respond to wavelengths between 0.7 μm and 1.0 μm, hence very-near infrared is also known as photographic infrared. Glass is opaque to infrared radiation of wavelength longer than 2 μm and other materials, such as germanium, quartz, and polyethylene, have to be used to make lenses and prisms.
- "not so excited" electrons in atoms, molecules, and semiconductors
light (a.k.a."visible light")
- roughly 400 - 700 nm
- a good, general sequence to remember is red, orange, yellow, green, blue, violet
- "hot" stuff
- "excited" electrons in atoms, molecules, and semiconductors
ultraviolet (a.k.a. "ultraviolet light")
- "beyond" violet
- a pigment of the imagination
- "very hot" stuff
- "very excited" electrons in in atoms, molecules, and (are we there yet?) semiconductors
- discovered in 1801
- moderately energetic, accelerated electric charges (just under ten to thousands of electron volts)
- classification I
  - Near ultraviolet occurs at wavelengths between 400 nm and 300 nm,
  - middle ultraviolet between 300 nm and 200 nm (the ozone layer of the atmosphere absorbs all wavelengths shorter than 290 nm)
  - extreme ultraviolet (or vacuum ultraviolet as absorption by the oxygen in the air makes the use of evacuated apparatus essential) between 200 nm and 150 nm.
- classification II, according to its effects on the skin
  - UVA (320-400 nm) The longest-wavelength range, UV -A, is not harmful in normal doses and is used clinically in the treatment of certain skin complaints, such as psoriasis. It is also used to induce vitamin D formation in patients that are …
  - UVB (290-320 nm) UVB rays are the tanning and cancer-causing rays.
  - UVC (100?230?-290 nm) UVC is is entirely absorbed by the ozone layer in the atmosphere and does not reach the earth's surface.
x-rays
- range of wavelengths is 10⁻¹¹ m to 10⁻⁹ m
- discovered in 1895
- energetic, accelerated electric charges (thousands to millions of electron volts)
  - bremsstrahlung -- braking acceleration
  - synchrotron -- centripetal acceleration
- extreme electron transitions to replace electrons dislodged from deep shells, very near the nucleus (bombardment of atoms by high-quantum-energy particles)
- soft x-rays, hard x-rays
gamma rays
- ranges in energy from about 10⁻¹⁵ to 10⁻¹⁰ joule (10 keV to 10 MeV) corresponding to a wavelength range of about 10⁻¹⁰ to 10⁻¹⁴ meter
- discovered in 1900
- very energetic, accelerated electric charges (millions to billions of electron volts and higher)
  - usually extraterrestrial in origin. High-energy particles that fall on the earth from space. Primary cosmic rays consist of nuclei of the most abundant elements, with protons (hydrogen nuclei) forming by far the highest proportion; electrons, positrons, neutrinos, and gamma ray photons are also present. The particle energies range from 10⁻¹¹ J to 10¹ J (10⁸ to 10²⁰ eV) and as they enter the earth's atmosphere they collide with oxygen and nitrogen nuclei producing secondary cosmic rays. The secondary rays consist of elementary particles and gamma -ray photons. A single high-energy primary particle can produce a large shower of secondary particles. The sources of the primary radiation are not all known, although the sun is believed to be the principal source of particles with energies up to about 10¹⁰ eV. It is believed that all particles with energies of less than 10¹⁸ eV originate within the Galaxy.
- nuclear reactions; excited nuclei returning to their ground state
  - usually terrestrial in origin

Summary

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Problems

practice

Write something.
- Answer it.
Write something.
- Answer it.
Write something.
- Answer it.
Write something completely different.
- Answer it.

numerical

problems

Resources

frequency allocation
- United Kingdom
  - UK Radio Frequency Bands
- United States
  - National Telecommunications and Information Administration
    - Frequency Allocation Chart
  - North American Railroad Radio Frequencies, Jon Roma
  - Committee on Radio Frequencies, National Academies
low frequency
- An Evaluation of the U.S. Navy's Extremely Low Frequency Submarine Communications Ecological Monitoring Program, National Academy of Sciences, 1999
- ELF transmitters on Google Earth
  - Clam Lake, Wisconsin, USA (ELF-WTF)
  - Republic, Michigan, USA (ELF-MTF)
  - Kola Peninsula, Russia (Зевс or Zevs or Zeus depending on the source)
- Loresco, deep AC grounding systems for the Navy's ELF system
- Radio Waves Below 22 kHz, Renato Romero
- VLF Group, Stanford University
  - Reception of ELF Signals at Antipodal Distances [pdf], Antony C. Fraser-Smith and Peter R. Bannister
- Stephen P. McGreevy's ground-based ELF-VLF recordings, University of Iowa
high frequency
- Major Edwin Howard Armstrong: The Inventor of FM, Chris Tsakis, WFMU
microwaves
- Imaging Radar (NASA/JPL)
- The Magnetron, Naval Electrical Engineering Training Series (NEETS), Integrated Publishing
- Microwaves Damage Food, Dr. Karl Kruszelnicki, Great Moments in Science, Australian Broadcasting Corporation
- Radar Meteorology: online remote sensing guide
- US Patent 2495429, Microwave Oven, Percey Spencer
millimeter waves
- Farran Technologies, manufacturers of millimeter wave technologies
- J.C. Bose: 60 GHz in the 1890s
- Millimeter Wave Technology Group, Rutherford-Appleton Laboratory
nonionizing radiation
- Health Protection Agency | Radiation Protection Division
- International Commission on Non-Ionizing Radiation Protection (ICNIRP)
radioastronomy

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