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The term electromagnetic spectrum refers to the collection of possible wavelengths of electromagnetic radiation. Radiation at a particular
wavelength λ (in vacuum) has an associated frequency ν and photon energy
E. These quantities are related according to the equations:
λ = c / ν
and
E = hν
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 radio waves at the long-wavelength end to gamma radiation at
the short-wavelength end, covering wavelengths from several 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.
Classifications
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 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.
Radio Waves
Radio waves cover the low-frequency, long-wavelength end of the spectrum. It is 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
typically 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.
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:
- 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)
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.
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
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