Electromagnetic spectrum
From Wikinfo
[[es:Espectro electromagn�tico]] [[fi:S�hk�magneettinen spektri]] [[fr:Spectre �lectromagn�tique]] [[fi:s�hk�magneettinen spektri]]
The term electromagnetic spectrum refers to the collection of possible wavelengths of electromagnetic radiation.
Contents |
Equations
Electromagnetic energy at a particular wavelength λ (in vacuum) has an associated frequency ν and photon energy E. These quantities are related according to the equations:
- <math>\lambda = c/\nu</math>
and
- <math>E=h\nu</math>
where c is the speed of light (3×108 m/s; wavelength times frequency): 300 Mm/s; 300 MmHz) and h = 6.65 × 10−34 J·s is Planck's constant, or, in alternative units, h = 4.1 μeV/GHz (photon; 4.1 feV per Hz). The wavelength times energy per photon is 1.24 μeVm.
Classifications
The electromagnetic spectrum 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. Radio, representing wavelengths from a few feet to well over a mile, is at one end of the spectrum. Gamma ray radiation is at the other end, so-called hard radiation: the wavelength of the harder types is so short, in the subatomic range, that we do not have instruments capable of directly measuring it.
| Image:Spectrum.png |
| Legend: γ = Gamma rays |
While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic radiation. For example 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 most 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.
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 grids. 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 is at the weak end of the spectrum, with low energy and long wavelength. It's 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.
Radio waves generally involve antennas of reasonable size, so their wavelengths range from hundreds of meters to about one millimeter. Different parts of the radio spectrum are called bands. Common names for the different bands may refer to the wavelength, relative frequency or the type of transmission commonly used. They are used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use Radio Waves.
|
Radio spectrum |
Extreme Low Frequency (ELF) bandwidth
Very Low Frequency (VLF)
Longwave
Mediumwave (AM)
Often called the AM band because it is used for short range AM broadcasting.
Shortwave (HF or high frequency)
Use was popularized when it was discovered that high frequency radio waves could skip long distances with relatively little power and smaller antennas including directional antennas. The most popular transmission formats are AM broadcasts and Single Side Band two-way communications.
Very High Frequency (VHF)
Includes the 2-meter band and American television channels 2-13.
Ultra High Frequency (UHF)
Includes the 4-meter band and American television channels 14-69.
Microwaves
The extremely high frequency (EHF) of Microwaves come next. They can cause entire molecules to resonate. This resonance causes water to move rapidly and enables the microwave oven to cook food. Low intensity microwave radiation is used in Wi-Fi.
Between 300 GHz and the mid-infrared, the absorption of electromagnetic radiation by molecular vibration of water in the Earth's atmosphere is so great that the atmosphere is effectively opaque to electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window freqency ranges. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy.
It should be noted that the average Microwave oven 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.
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.
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.
Infrared radiation
The next category is infra-red. This makes chemical bonds resonate. When a chemical bond resonates, the vibrations add internal energy to the molecule. The molecule becomes hot. The bulk substance becomes hot when its molecules' bonds are all resonating. When you touch it, you feel its warmth or you lose the tip of your finger, depending on how violent the resonance is.
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:
- Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth's atmosphere absorbs so strongly in this range that in renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimeter" in astronomy, reserving far infra-red for wavelengths below 200 μm.
- Mid-infrared, from 30 THz (10 μm) to 120 THz (2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, that is, when the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.
- Near-infrared, from 120 THz (2.5 μm) to 400 THz (750 nm). Physical processes that are relevant for this range are similar to those for visible light.
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 |
After infra-red comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. When this is scattered or reflected by an object, we can infer the existence of the object. A person can see the light scattered from his or her room's light by his or her keyboard, so his or her brain infers that the keyboard exists. 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.
Since current computer screens only use 3 primary colours, only the red, green and blue actually consist of single colours in the image, the rest is composite.
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.
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.
See also
External links
- U.S. Frequency Allocation Chart - Covering the range 3kHz-300GHz (from Department of Commerce)
- Canadian Table of Frequency Allocations
- UK frequency allocation table (from the Radiocommunications Agency, pdf format)
Adapted from the Wikipedia article, "Electromagnetic_spectrum", used under the GNU Free Documentation License
