Electromagnetic Spectrum

Glenn Elert

Electromagnetic Spectrum

Discussion

introduction

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

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

Radio Frequency Bands
name		ITU¹ number		frequency			wavelength
extremely low frequency	(ELF)*	1	(~10⁰¹ Hz)	3	-	30 Hz	100,000	-	10,000 km
super low frequency	(SLF)*	2	(~10⁰² Hz)	30	-	300 Hz	10,000	-	1,000 km
ultra low frequency	(ULF)*	3	(~10⁰³ Hz)	300	-	3000 Hz	1000	-	100 km
very low frequency	(VLF)	4	(~10⁰⁴ Hz)	3	-	30 kHz	100	-	10 km
low frequency	(LF)^†	5	(~10⁰⁵ Hz)	30	-	300 kHz	10	-	1 km
medium frequency	(MF)^†	6	(~10⁰⁶ Hz)	300	-	3000 kHz	1000	-	100 m
high frequency	(HF)^†	7	(~10⁰⁷ Hz)	3	-	30 MHz	100	-	10 m
very high frequency	(VHF)	8	(~10⁰⁸ Hz)	30	-	300 MHz	10	-	1 m
ultra high frequency	(UHF)	9	(~10⁰⁹ Hz)	300	-	3000 MHz	1000	-	100 mm
super high frequency	(SHF)	10	(~10¹⁰ Hz)	3	-	30 GHz	100	-	10 mm
extremely high frequency	(EHF)	11	(~10¹¹ Hz)	30	-	300 GHz	10	-	1 mm
tremendously high frequency	(THF)^‡	12	(~10¹² Hz)	0.3	-	3 THz	1	-	0.1 mm

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
  
  Magnify
  
  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.

Radar frequency bands
		frequency range (GHz)
	band	IEEE¹			ITU²			ISO³
P	previous							0.225	–	0.39
L	long	1	–	2	1.215 1.525	– –	1.400 1.710*	0.39	–	1.55
S	short	2	–	4	2.300 2.500 2.700	– – –	2.500 2.690* 3.400	1.55	–	5.20
C	compromise	4	–	8	3.400 4.500 5.250 5.850	– – – –	4.200* 4.800* 5.850 7.075*	3.90	–	6.20
X	crosshair	8	–	12	8.500	–	10.50	5.20	–	10.9
_uK_u	kurze under	12	–	18	10.70 13.40 14.00 15.30	– – – –	13.25* 14.00 14.50* 17.30
K	kurze^‡	18	–	27	17.70 24.05 24.65	– – –	20.20* 24.25 24.75^†	10.9	–	36
_aK_a	kurze above	27	–	40	27.50 33.40	– –	30.00* 36.00
Q								36	–	46
V		40	–	75	37.50 47.20 59.00	– – –	42.50* 50.20* 64.00^†	46	–	56
W		75	–	110	76.00 92.00	– –	81.00^† 100.0^†	56	–	100
mm	millimeter	110	–	310	126.0 144.0 231.0 238.0	– – – –	142.0^† 149.0^† 235.0^† 248.0^†

Earth Exploration-Satellite Services (Active) Reserved Frequencies
frequency (GHz)	required bandwith
frequency (GHz)	scatterometer	altimeter	synthetic aperture radar	precipitation radar	cloud profiling radar
0.432–0.438			6 MHz
1.215–1.300	5–500 kHz		20–085 MHz
3.100–3.300		200 MHz	20–200 MHz
5.250–5.570	5–500 kHz	320 MHz	20–320 MHz
8.550–8.650	5–500 kHz	100 MHz	20–100 MHz
9.300–9.900	5–500 kHz	300 MHz	20–600 MHz
13.25–13.75	5–500 kHz	500 MHz		0.6–14 MHz
17.20–17.30	5–500 kHz			0.6–14 MHz
24.05–24.25				0.6–14 MHz
35.50–36.00	5–500 kHz	500 MHz		0.6–14 MHz
78.00–79.00					0.3–10 MHz
94.00–94.10					0.3–10 MHz
133.5–134.0					0.3–10 MHz
237.9–238.0					0.3–10 MHz

Radio Astronomy Service (Passive) Reserved Frequencies
frequency range (GHz)			notes
1.350	–	1.400	protected at 15 geographic areas in the US*
1.400	–	1.427	H (1.420406 GHz)
1.718.8	–	1722.2	protected at 15 geographic areas in the US*
2.690	–	2.700
4.590	–	4.990	protected at 15 geographic areas in the US*
10.68	–	10.70
15.35	–	15.40
23.60	–	24.00	NH₃ (23.6946 GHz, 23.7236 GHz, 23.8706 GHz)
31.30	–	31.50
31.50	–	31.80
48.94	–	49.04
50.20	–	50.40
52.60	–	54.25
86.00	–	92.00	clouds, oil spills, ice, snow; HCN (88.632 GHz)
100.00	–	102.00	NO (100.49 GHz)
109.50	–	111.80	CO (109.782 GHz, 110.201 GHz, 112.35 GHz), O₃ (110.8 GHz)
114.25	–	116.00	CO (115.221 GHz, 115.271 GHz)
148.50	–	151.50	surface, water vapor, cloud parameters; NO (150.74 GHz)
164.00	–	167.00	Cloud water, rain, ice; ClO (164.38 GHz, 167.2 GHz)
182.00	–	185.00	H₂O (183.31 GHz), O₃ (184.75 GHz)
190.00	–	191.80	three-dimensional sounding for water vapor
200.00	–	209.00	NO (200.98 GHz)
226.00	–	231.50	CN (226.600 GHz, 226.800 GHz), CO (230.538 GHz)
250.00	–	252.00	NO (250.62 GHz, 51.21 GHz)

infrared (a.k.a. "infrared light")

"below" red
a pigment of the imagination
radiation in the wavelength range 0.7 micrometer to 1 millimeter
- Infrared radiation (IR radiation) is em radiation for which the wavelengths are longer than those for visible radiation.
- For infrared radiation, the range between 780 nm and 1 mm is commonly subdivided into:
  - IR-A: 780 nm to 1400 nm (0.78 μm to 1.4 μm)
  - IR-B: 1.4 μm to 3.0 μm
  - IR-C: 3 μm to 1,000 μm (0.003 mm to 1 mm)
- A precise border between visible radiation and infrared radiation cannot be defined because visual sensation at wavelengths greater than 780 nm can be experienced.
- In some applications the infrared spectrum has also been divided into "near", "middle", and "far" infrared; however, the borders necessarily vary with the application.
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 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.
- far-infrared a.k.a. terahertz waves
  - terahertz radiation is estimated to account for 98% of all the photons that have been emitted since the big bang — P.H. Siegel, IEEE Transactions Microwave Theory Technology, 30, 910 (2002)
  - 0.8–4 THz known as the terahertz gap, frequencies just below the reach of optical technologies and just above the reach of electronics
- 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
- Light is radiation that is considered from the point of view of its ability to excite the visual system. The term "light" is sometimes used for optical radiation extending outside the visible range, but this usage is not recommended.
- Visible radiation is capable of causing a visual sensation directly. There are no precise limits for the spectral range of visible radiation since they depend upon the amount of radiant flux reaching the retina and the responsivity of the observer. The lower limit is generally taken between 360 nm and 400 nm and the upper limit between 760 nm and 830 nm.
a good, general sequence to remember is red, orange, yellow, green, blue, violet
"hot" stuff; incandescence, incandescent
"excited" electrons in atoms, molecules, and semiconductors; luminescence, luminescent

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 electronvolts)
classification I (ISO 21348)
- NUV, near ultraviolet (300–400 nm)
- MUV, middle ultraviolet (200–300 nm)
  The ozone layer of the atmosphere absorbs all wavelengths shorter than 290 nm.
- FUV, far ultraviolet (122–200 nm)
- EUV, extreme ultraviolet (10–121 nm)
  It should really be XUV since the word extreme is alliterative with the letter x — as in x-rays. Extreme ultraviolet radiation is verging on x-rays.
- VUV, vacuum ultraviolet (10–200 nm)
  Absorption by the oxygen in air makes the use of evacuated apparatus essential when working with FUV and EUV radiation.
radiation in the wavelength range 100 nm to 400 nm
- Ultraviolet radiation (UV radiation) is optical radiation for which the wavelengths are shorter than those for visible radiation
- The range between 100 nm and 400 nm is commonly subdivided into:
  - UV-A: 315 nm to 400 nm
  - UV-B: 280 nm to 315 nm
  - UV-C: 100 nm to 280 nm
- A precise border between "ultraviolet radiation" and "visible radiation" cannot be defined, because visual sensation at wavelengths shorter than 400 nm is noted for very bright sources.
- In some applications the ultraviolet spectrum has also been divided into "far," "vacuum," and "near" ultraviolet; however, the borders necessarily vary with the application (e.g. in meteorology, optical design, photochemistry, thermal physics).
classification II (ISO 21348), according to its effects on the skin, Photobiological designations of the Commission Internationale de l'Eclairage (International Commission on Illumination), cited in IARC 1992.
- UVA (315–400 nm)
  UVA is the dominant tanning ray and plays a major part in skin aging and wrinkling (photoaging). "Most of us are exposed to large amounts of UVA throughout our lifetime. UVA rays account for up to 95 percent of the UV radiation reaching the Earth's surface. Although they are less intense than UVB, UVA rays are 30 to 50 times more prevalent. They are present with relatively equal intensity during all daylight hours throughout the year, and can penetrate clouds and glass." Black light. Maximum Permissible Exposure (MPE) 1,000 μW/cm² 8 hours. Suntans are related to UVA exposure. They do not cause sunburns because of their lower energy than UVB or UVC. Since UVA penetrate deeper they damage collagen fibers and destroy vitamin A. Some skin problems (psoriasis, for example) can be treated with UV light. For a treatment known as PUVA, a drug called a psoralen is given first. The drug collects in the skin and makes it more sensitive to UV. Then the patient is treated with UVA radiation. Another treatment option is the use of UVB alone (without a drug).
- UVB (280–315 nm)
  The tanning and cancer-causing rays. Erythemal — the chief cause of skin reddening and sunburn. MPE 500 μW/cm² 1 minute. Photokeratitis, Welder's Flash, or Arc Eye is literally burning of the cornea by intense exposure to UVB. Cataracts, pterygia, pinguecula. One positive affect of moderate doses of UVB is that in induces the production of vitamin D and vitamin K. Used clinically in the treatment of certain skin complaints (such as psoriasis) and to induce vitamin D formation in patients that are deficient. Acne, psoriasis, neonatal high levels of bilirubin, and daylight deprivation depression are some of the ailments treated with UVB.
- UVC (100–280 nm)
  This radiation band is entirely absorbed by the ozone layer in the atmosphere and does not reach the Earth's surface. Germicidal. MPE 100 μW/cm² 1 minute.

x-rays

range of wavelengths is 10⁻¹¹ m to 10⁻⁹ m
discovered in 1895
energetic, accelerated electric charges (thousands to millions of electronvolts)
- bremsstrahlung — braking acceleration
- synchrotron — centripetal acceleration
extreme electron transitions to replace electrons dislodged from deep shells 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 electronvolts 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