Editing Refractive index (section)

===Refractivity===
In atmospheric applications, '''refractivity''' is defined as {{math| ''N'' {{=}} ''n'' – 1}}, often rescaled as either<ref>{{cite web | last = Young | first = A.T. | year = 2011 | title = Refractivity of Air | url = http://mintaka.sdsu.edu/GF/explain/atmos_refr/air_refr.html | access-date = 31 July 2014 | url-status = live | archive-url = https://web.archive.org/web/20150110053602/http://mintaka.sdsu.edu/GF/explain/atmos_refr/air_refr.html | archive-date = 10 January 2015 }}</ref> {{math|''N'' {{=}} {{10^|6}} (''n'' – 1)}}<ref>{{cite journal | last1 = Barrell | first1 = H. | last2 = Sears | first2 = J.E. | year = 1939 | title = The refraction and dispersion of air for the visible spectrum | journal = [[Philosophical Transactions of the Royal Society of London]] | series =  A, Mathematical and Physical Sciences | volume = 238 | issue = 786 | pages = 1–64 | jstor = 91351 | doi=10.1098/rsta.1939.0004 | doi-access = free |bibcode = 1939RSPTA.238....1B }}</ref><ref>{{cite journal |last1=Aparicio |first1=Josep M. |last2=Laroche |first2=Stéphane |date=2011-06-02 |df=dmy-all |title=An evaluation of the expression of the atmospheric refractivity for GPS signals |journal=[[Journal of Geophysical Research]] |volume=116 |issue=D11 |page=D11104 |doi=10.1029/2010JD015214 |doi-access=free |bibcode=2011JGRD..11611104A }}</ref> or {{math|''N'' {{=}} {{10^|8}} (''n'' – 1)}};<ref>{{cite journal | last = Ciddor | first = P.E. | year = 1996 | title = Refractive index of air: New equations for the visible and near infrared | journal = Applied Optics | volume = 35 | issue = 9 | pages = 1566–1573 | doi=10.1364/ao.35.001566 | pmid = 21085275 |bibcode = 1996ApOpt..35.1566C }}</ref> the multiplication factors are used because the refractive index for air, {{mvar|n}} deviates from unity by at most a few parts per ten thousand.

''[[Molar refractivity]]'', on the other hand, is a measure of the total [[polarizability]] of a [[mole (unit)|mole]] of a substance and can be calculated from the refractive index as
<math display="block">A = \frac{M}{\rho} \cdot \frac{n^2 - 1}{n^2 + 2}\ ,</math>
where {{mvar|ρ}} is the [[density]], and {{mvar|M}} is the [[molar mass]].<ref name=bornwolf/>{{rp|93}}