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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)

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.






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

There are several terms used.
== See also ==
* [[Electromagnetic fields and waves]]

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