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Forbidden transitions are shown in green → forbidden transitions in the visible spectrum are shown in green. (The ones in red are forbidden also, but they are not in the visible.) |
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[[File:Oiii-linesp.svg|thumb|right|A [[Grotrian diagram]] of doubly ionized oxygen: forbidden transitions in the visible spectrum are shown in green.]]
In [[astronomy]] and [[atomic physics]], '''doubly ionized oxygen''' (also known as '''O III''') is the [[ion]] [[oxygen|O]]<sup>2+</sup>. Its emission [[forbidden line]]s in the [[visible spectrum]], primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm, are known in [[astronomical spectroscopy]] as '''[O III]'''. Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new [[chemical element]]. Concentrated levels of O III are found in [[diffuse nebula|diffuse]] and [[planetary nebula|planetary]] nebulae. Consequently, narrow [[Dichroic filter|band-pass filter]]s that isolate the 501 nm and 496 nm wavelengths of light, that correspond to [[green]]-[[turquoise (color)|turquoise]]-[[cyan]] [[spectral colors]], are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly [[light pollution|light-polluted]] terrestrial atmosphere) where the frequencies of [O III] are much less pronounced.
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