Electromagnetic spectrum
| Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency MF = Medium frequency LF = Low frequency VLF = Very low frequency VF = Voice frequency ELF = Extremely low frequency |
and
where c is the speed of light (3×108 m/s) and h = 6.65 × 10−34 J·s is Planck's constant, or, in alternative units, h = 4.1 μeV/GHz.
The electromagnetic spectrum, shown in the table, extends from electric power at the long-wavelength end to gamma radiation at the short-wavelength end, covering wavelengths from thousands of miles down to fractions of the size of an atom.
In the branch of physics called electromagnetic spectroscopy, the spectra of radiation absorbed and emitted by matter is used to obtain information about matter.
While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60Hz may be received and studied by astronomers, or may be ducted along wires as Electric Power. Also, some low-energy gamma rays actually have a longer wavelength than some high-energy X-rays. This is possible because "gamma ray" is the name given to the photons generated from nuclear decay or other nuclear and subnuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma ray and X-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays. However, there are a few low-energy nuclear transitions (eg. the 14.4 keV nuclear transition of Fe-57) that produce gamma rays that are less energetic than some of the higher energy X-rays.
Use of the radio frequency spectrum is regulated by governments.
This is called frequency allocation.
It should be noted that an average Microwave oven in active condition is, in close range, powerful enough to cause interference with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.
Currently no efficient sources exist for microwave energy at the high end of the band, sub-millimeter waves or so-called terahertz waves, so this portion of the EM spectrum is relatively unused at present.
After infrared comes visible light.
This is the range in which the sun and stars similar to it emit most
of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.
Note that there are no defined boundaries between the types of electromagnetic radiation. Some wavelengths have a mixture of the properties of two regions of the spectrum. For example, red light resembles infra-red radiation in that it can resonate some chemical bonds.
Classifications
Electric power
Electric Power covers the low-frequency, long-wavelength end of the spectrum. The radiation is usually ducted along 2-wire and 3-wire transmission lines and sent to various devices besides antennas. At zero frequency the energy is emitted by batteries and DC power supplies, while at 50Hz and 60Hz it is produced by rotary magnetic generators and ducted through the international power gridss. At frequencies between 20Hz - 30KHz the EM energy is translated to and from acoustic energy and is distributed over telephone lines, as well as being used to operate loudspeakers for public address or in music systems. Note that other than its frequency, there is no physical difference between the VHF energy guided along a television coaxial cable, versus the 60Hz travelling along the cord leading to a light bulb. When connected to the appropriate antenna, both will radiate into space.Radio Waves
Radio waves generally involve antennas of reasonable size, so their wavelengths are limited to hundreds of meters and shorter. They are used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use it. Radio Waves can be detected at the Ultra High Frequency (UHF), Very High Frequency (VHF), Shortwave (HF or high frequency), Medium Wave (AM), Longwave, Very Low Frequency (VLF), and Extreme Low Frequency (ELF) bandwidth.Microwaves
The extremely high frequency (EHF) of Microwaves come next. Microwaves are waves which are typically short enough to employ tubular metal waveguides of reasonable diameter. Microwave energy is produced with Klystron and Magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi.Infrared radiation
The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:Visible radiation (light)
.
Color
Wavelength interval
Frequency interval
red
~ 625-740 nm
~ 480-405 THz
orange
~ 590-625 nm
~ 510-480 THz
yellow
~ 565-590 nm
~ 530-510 THz
green
~ 520-565 nm
~ 580-530 THz
cyan
~ 500-520 nm
~ 600-580 THz
blue
~ 450-500 nm
~ 670-600 THz
violet
~ 380-430 nm
~ 790-700 THz
Continuous spectrum
The spectrum of visible light
Ultraviolet light
Next comes ultraviolet. This is radiation whose wavelength is shorter than the violet end of the visible spectrum. It was discovered to be useful for astronomy by a Mariner probe at Mercury, which detected UV that "had no right to be there". The dying probe was turned over to the UV team full time. The UV source turned out to be a star, but UV astronomy was born. Being very energetic, UV can break chemical bonds. Chlorine will not normally react with an alkane, but give it UV and it reacts quickly.
This is because the UV breaks the bond holding chlorine atoms into molecules of Cl2. Lone atoms are extremely reactive and will react with the otherwise almost-inert alkanes. It also makes a mess of DNA, causing cell death at best and uncontrolled cell reproduction (cancer) at worst.X-rays
After UV come X-rays. Hard X-rays are of shorter wavelengths than soft X-rays. X-rays are used for seeing through some things and not others, as well as for high-energy physics and astronomy. Black holes and neutron stars emit x-rays, which enable us to study them.Gamma rays
After hard X-rays come gamma rays. These are the most energetic photons, having no lower limit to their wavelength. They are useful to astronomers in the study of high-energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.See also
External links
| Electromagnetic Spectrum
Radio waves | Microwave | Infrared | Visible light | Ultraviolet | X-ray | Gamma ray Visible: Red | Orange | Yellow | Green | Blue | Indigo | Violet |
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Radio spectrum ELF | SLF | ULF | VLF | LF/LW | MF/MW | HF/SW | VHF | UHF | SHF | EHF 3Hz | 30Hz | 300Hz | 3kHz | 30kHz | 300kHz | 3MHz | 30MHz | 300MHz | 3GHz | 30GHz | 300GHz |