Difference between revisions 990114 and 990115 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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== Visible light ==
This is of course the most familiar form of electromagnetic radiation.  It covers wavelengths of around 400-700nm (frequencies of 430-750 THz); different people have different limits.  The 
longest wavelengths correspond to red, then orange, yellow, green, blue, indigo and violet are shorter and shorter.

It has been claimed that some people who have had cataracts removed can see shorter wavelengths than other people, because they get replacement lenses in their eyes that are more transparent torange of wavelengths that a person can detect has is nothing to do with colour blindness, which refers to the ability to perceive differences in frequency as colour rather than ability to detect the radiation.

The longest wavelengths correspond to red, then orange, yellow, green, blue, indigo and violet are shorter and shorter.  There is no sharp division into different colours; they merge gradually into each other as these wavelengths than natural len increases.

This is a relatively narrow band, covering a factor of no more than two in wavelength or frequency.  However, it is comparatively well defined.

The exact frequencies that register as visible light vary from person to person.  For example, there are several cases of astronomers who claimed that following a cataract operation, they could see shorter wavelengths than other people, slightly into the ultraviolet (e.g. [http://ehilbert.wso.net/Starlight/deep_sky_wonders.htm Walter Scott Houston]).  This is nothing to do with colour blindness, which refers to the ability to perceive differences in frequency as colour rather than ability to detect the radiationIt has been reported that some people who have had cataracts removed can see shorter wavelengths than other people, because they get replacement lenses in their eyes that are more transparent to these wavelengths than natural lenses.  A well-documented case is the astronomer [http://ehilbert.wso.net/Starlight/deep_sky_wonders.htm Walter Scott Houston].

== Ultraviolet (UV) ==
'''Ultraviolet radiation''' was discovered by Johann Wilhelm Ritter in 1801.

There are several terms used.

* '''Near ultraviolet''': 400-300nm
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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]]