Difference between revisions 990113 and 990114 on enwikiversity

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'''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.

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== Infra-red ==
'''Infra-red''' radiation, the first non-visible electromagnetic radiation to be discovered, was discovered by William Herschel in 1800.

It has wavelengths below 1mm, down to where they start to become visible (about 700nm).  Thus the frequency range is from 300 GHz up to about 430 THz.  Shorter wavelengths are '''near infra-red'''; middle ones are '''mid infra-red'''
; longer ones are '''far infra-red'''.

The longest wavelengths (0.1-1mm, 300GHz-3THz) are sometimes regarded as a distinct band called '''Submillimetre radiation''' (from their wavelength) or '''Terahertz radiation''' (from their frequency).  They can penetrate through some objects opaque to visible light, but can be used to produce images, so are a safe alternative to some uses of X-rays.

== Visible light ==
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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]

== See also ==
* [[Electromagnetic fields and waves]]

[[Category:Resources last modified in April 2012]]
[[Category:Physics]]