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Electromagnetic radiation
'''Electromagnetic radiation''' comes in many different types, although the differences between them are quantitative rather than qualitative.  This teaching aid lists the different types that are generally recognised.

Electromagnetic radiation is an Electromagnetic oscillation wave that travels at speed of visible light carries[[Quantum]]'s energy called [[Photon]]

==Electromagnetic oscillation wave==
(contracted; show full)

There are many bands within these, which are even vaguer and more prone to overlap.  Each band is discussed in more detail below.  The limits given are rough, so exact consistency between wavelength, frequency and photon energy cannot be expected.

⏎
==Units==
{{main|Units}}
Scientists usually use SI units.  The unit of frequency is the hertz (Hz), one cycle per second, named after Heinrich Hertz.  For our purposes, we shall use much higher frequencies.  One thousand Hz is a kHz (kilohertz); one thousand kHz is a MHz (megahertz); one thousand MHz is a GHz (gigahertz); one thousand GHz is a THz (terahertz).

The SI unit of length is the metre (m).  One thousand metres is a kilometre (km).  One hundredth of a metre is a centimetre (cm); one thousandth is a millimetre (mm), one millionth is a micrometre (&mu;m, often called a micron and written &mu;).  A thousandth of a micrometre is called a nanometre (nm).  A tenth of a nm is known as an Angstrom unit or Angstrom (Å).  The use of cm and Angstrom is discouraged in the SI system, but many scientists like them.

'''Exercise:''' Find out why the cm and Angstrom are discouraged in the SI system.

The SI unit of energy is the Joule (J).  However, there is a much smaller unit, the electron volt (eV).  Again, its use is discouraged in the SI system, but many scientists like it.  The eV is the energy gained by an electron in passing through a potential difference of one volt.  Since the charge on an electron is 1.60218 x 10<sup>-19</sup> Coulombs, an eV is 1.60218 x 10<sup>-19</sup> J.  A keV is 1000 eV and a MeV is 1000 keV.

As will be discussed below, either frequency or wavelength may be used for radio waves.  Wavelength is usually used for infrared, visible and ultraviolet (expressed in nm or Å), and photon energy for X-rays and gamma rays (expressed in keV or MeV).

'''Exercise:''' Find out why the eV is discouraged in the SI system.  What would be an appropriate SI unit to replace the keV or MeV?  Why are these units more convenient for physicists than the SI ones?

== Relationships ==
By definition, wavelength x frequency = speed.  Since the 

speed of light c = 2.997458 x 10<sup>8</sup> m/s, we have

:&lambda; &nu; = c

Also, by Planck's law,

:E = h &nu;

where h is Planck's constant 6.62607 x 10<sup>-34</sup> Js.

It follows that a photon with an energy of 1eV has a frequency of 1 eV/h = 2.41799 x 10<sup>14</sup> Hz or about 242 THz and a wavelength of c.h/1 eV = 1.23984 x 10<sup>-6</sup> m or about 1,240nm or 12,400Å.  As will be seen below, that would put the photon in the infrared range.  In practice, photon energies are never quoted for such long wavelengths.

'''Exercise:''' Find why photon energies are only important at very short wavelengths.



==Radio rays==
{{main|Radiation/Radios|Radio radiation|Radios}}
The existence of '''radio waves''' was predicted by James Clerk Maxwell in 1864. Their properties were investigated by Heinrich Hertz (after whom the hertz is named) in 1885-9.

With the splitting-off of microwaves as a separate band, radio waves are now regarded as having wavelengths exceeding 30cm, frequencies below 1GHz.  There is no upper limit to the wavelength hence no lower limit to the frequency.  The standard divisions are:

* '''Ultra high frequency (UHF)''': Frequencies of 300MHz to 3GHz, hence wavelengths of 10cm to 1m.  This overlaps with the microwave region.
* '''Very high frequency (VHF)''': Frequencies of 30MHz to 300MHz, hence wavelengths of 1m to 10m.
* '''High frequency (HF)''': Frequencies of 3MHz to 30MHz, hence wavelengths of 10m to 100km.
* '''Medium frequency (MF)''': Frequencies of 300kHz to 3MHz, hence wavelengths of 100m to 1km.  These are also called '''hectometric waves''' (hectometre = 100m).
* '''Low frequency (LF)''': Frequencies of 30kHz to 300kHz, hence wavelengths of 1km to 10km.
* '''Very low frequency (VLF)''': Frequencies of 10kHz to 30kHz, hence wavelengths of 10km to 30km.
* '''Ultra low frequency (ULF)''': Frequencies of 300Hz to 10kHz, hence wavelengths of 30km to 1000km; these are mainly used for long-distance underwater transmission.
* '''Extremely low frequency (ELF)''': The lowest frequencies (below 300 Hz; wavelength > 1000km).

Sometimes a division by wavelength is used:

* '''Short wave (SW)''': Wavelengths of 10-200m hence frequencies of 1.5-33MHz; there are several sub-bands.
* '''Medium wave (MW)''': Wavelengths of 200-1000m hence frequencies of 300kHz-1.5MHz.
* '''Long wave (LW)''': Wavelengths > 1000m hence frequencies < 300kHz.

Other terms found are:

* '''Frequency modulation (FM)''': This is not a frequency band, but a way of coding the audio signal onto a radio wave.  However, quite a high frequency is necessary for FM to work well, and in practice frequencies of close to 100MHz (wavelength 3m, the middle of the VHF band) are used.

* '''Digital radio''': Again, this is not a frequency band.  Typically, frequencies of the order of 200MHz (wavelength 1.5m, towards the short wavelength or high frequency end of the VHF band) are used.

* '''Decametric''': Wavelengths of 10-30m (decametre = 10m) hence frequencies of 10-33MHz.  The planet Jupiter radiates strongly at these wavelengths.⏎
⏎
==Microwaves==
{{main|Radiation/Microwaves|Microwave radiation|Microwaves}}
'''Microwaves''' in radio telecommunications are the very shortest wavelengths usable for communication.  They have wavelength 1mm to 30cm, or frequency 1-300 GHz.

An important frequency is 1420 MHz (wavelength 21.1cm).  Interstellar hydrogen radiates at this frequency, making it a very useful frequency for radio astronomers.

(contracted; show full)

Gamma rays are even more dangerous to living things than are X-rays, and are sometimes used to sterilise equipment.

There is no lower boundary to the wavelength hence no upper bound to frequency or photon energy.  In October 2011, photons with energies exceeding 100 billion eV, i.e. over a million times the lower limit, were detected from the Crab pulsar.[http://www.sciencedaily.com/releases/2011/10/111006141358.htm]


==Units==
{{main|Units}}
Scientists usually use SI units.  The unit of frequency is the hertz (Hz), one cycle per second, named after Heinrich Hertz.  For our purposes, we shall use much higher frequencies.  One thousand Hz is a kHz (kilohertz); one thousand kHz is a MHz (megahertz); one thousand MHz is a GHz (gigahertz); one thousand GHz is a THz (terahertz).

The SI unit of length is the metre (m).  One thousand metres is a kilometre (km).  One hundredth of a metre is a centimetre (cm); one thousandth is a millimetre (mm), one millionth is a micrometre (&mu;m, often called a micron and written &mu;).  A thousandth of a micrometre is called a nanometre (nm).  A tenth of a nm is known as an Angstrom unit or Angstrom (Å).  The use of cm and Angstrom is discouraged in the SI system, but many scientists like them.

'''Exercise:''' Find out why the cm and Angstrom are discouraged in the SI system.

The SI unit of energy is the Joule (J).  However, there is a much smaller unit, the electron volt (eV).  Again, its use is discouraged in the SI system, but many scientists like it.  The eV is the energy gained by an electron in passing through a potential difference of one volt.  Since the charge on an electron is 1.60218 x 10<sup>-19</sup> Coulombs, an eV is 1.60218 x 10<sup>-19</sup> J.  A keV is 1000 eV and a MeV is 1000 keV.

As will be discussed below, either frequency or wavelength may be used for radio waves.  Wavelength is usually used for infrared, visible and ultraviolet (expressed in nm or Å), and photon energy for X-rays and gamma rays (expressed in keV or MeV).

'''Exercise:''' Find out why the eV is discouraged in the SI system.  What would be an appropriate SI unit to replace the keV or MeV?  Why are these units more convenient for physicists than the SI ones?

== Relationships ==
By definition, wavelength x frequency = speed.  Since the 

speed of light c = 2.997458 x 10<sup>8</sup> m/s, we have

:&lambda; &nu; = c

Also, by Planck's law,

:E = h &nu;

where h is Planck's constant 6.62607 x 10<sup>-34</sup> Js.

It follows that a photon with an energy of 1eV has a frequency of 1 eV/h = 2.41799 x 10<sup>14</sup> Hz or about 242 THz and a wavelength of c.h/1 eV = 1.23984 x 10<sup>-6</sup> m or about 1,240nm or 12,400Å.  As will be seen below, that would put the photon in the infrared range.  In practice, photon energies are never quoted for such long wavelengths.

'''Exercise:''' Find why photon energies are only important at very short wavelengths.⏎
⏎
⏎
⏎
⏎
== See also ==
* [[Electromagnetic fields and waves]]

==Ultraviolets==
{{main|Radiation/Ultraviolets|Ultraviolet radiation|Ultraviolets}}
'''Ultraviolet radiation''' was discovered by Johann Wilhelm Ritter in 1801.

There are several terms used.

==External links==
[[Category:Physics]]
[[Category:Electromagnetism]]
[[Category:Radiation]]
[[Category:Resources last modified in August 2016]]