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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.

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

==Electromagnetic oscillation wave==
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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.

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==Frequency band==
===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.

Microwaves have only fairly recently been regarded as distinct from very short wavelength radio waves.  Their recognition is probably due to the popularity of microwave ovens.  In fact, these ovens typically use frequencies of 300MHz to 1GHz, or wavelengths 30cm to 100cm, frequencies in the lower half of the UHF band hence slightly below the radio telecommunications definition.

===Infra-reds===
{{main|Radiation/Infra-reds|Infra-red radiation|Infra-reds}}
'''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.

===Visuals===
{{main|Radiation/Visuals|Visual radiation|Visuals|Visibles}}
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 range 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 the wavelength 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.

It 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===
* '''Near ultraviolet''': 400-300nm
* '''Middle ultraviolet''': 300-200nm
* '''Extreme or far ultraviolet''': 200-100nm

* '''Vacuum ultraviolet''' is UV with wavelengths shorter than about 150-200nm.  Such wavelengths are strongly absorbed by air, especially oxygen, so are best used in a vacuum.  However, it is possible to use pure nitrogen rather than a vacuum.

'''UVA, UVB, UVC''' are terms used to assess how dangerous the UV is to human skin.

* '''UVA''': 320-400 nm: Not absorbed by ozone layer; fairly harmless		
* '''UVB''': 280-320 nm: Partly absorbed by ozone layer; more dangerous		
* '''UVC''': 100-280 nm: Totally absorbed by ozone layer; most dangerous

Short-wave UV, X-ray and gamma-ray radiation are sometimes lumped together as '''ionising radiation''', because the photons are energetic enough to knock electrons out of atoms, ionising them.  Such radiation is dangerous to living things, because the ionisation can disrupt biochemical processes and even cause cancer.  Alpha and beta particles can also be called ionising radiation.

===X-rays===
{{main|Radiation/X-rays|X-ray radiation|X-rays}}
These have a wavelength of 0.01 to 10 nm, hence photon energies of 1keV to 1000keV.  They were discovered by Wilhelm Roentgen in 1895.

Often, X-rays are divided into '''hard X-rays''' (wavelength of 0.01 to 0.1 nm, photon energies of 10keV to 1000keV) and '''soft X-rays''' (wavelength of 0.1 to 10 nm, photon energies of 0.1keV to 10keV).  In general, shorter wavelength X-rays are said to be harder than longer wavelength ones.

X-rays are widely used in medicine and technology because they can penetrate many materials hence reveal internal structures.  Excessive exposure of living beings to X-rays, especially very hard ones, can be damaging and even fatal.  Long-term exposure to even moderate levels can cause cancer in humans.

===Gamma rays===
{{main|Radiation/Gamma rays|Gamma-ray radiation|Gamma rays}}
These have a wavelength of 0.01 nm or less, hence photon energies exceed 100keV.  They were discovered by Henri Becquerel in 1898 as a form of radioactivity.  (He discovered three types, which he called alpha, beta and gamma, hence the name.  However, alpha and beta are particles, not electromagnetic rays.)

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

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There are several terms used.

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