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Re: Refraction
From: Marcel Tschudin
Date: 2005 Aug 14, 21:09 +0300
From: Marcel Tschudin
Date: 2005 Aug 14, 21:09 +0300
Fred, Here the influence of humidity on the refractive index of air in figures: This page from ESO describes the procedure on how the index can be calculated: http://www.ls.eso.org/lasilla/sciops/2p2/E2p2M/FEROS/Projects/ADC/references/refraction/ The tabulated values on page 17 of the following manuscript from Stanford correspond to the "dry part" of the index, as veryfied with the above "procedure" for standard condition. http://www.stanford.edu/group/efmh/FAMbook/Chap10.pdf Finally Andrew Young shows on his Web page the refractivity of air at standard condition including the "wet part" as calculated by Peck and Reeder http://mintaka.sdsu.edu/GF/explain/optics/disp.html Now, what effect does this have on refraction? Using 0? altitude at sea level (height=0m), standard temperrature (=15?C) and pressure (=1013.25hPa) and a wavelength of 595nm (orange, e.g. sunrise or sunset). By integration over the light path: Index with "dry part" only =1.000277 -> Refraction = 30.27 minutes Index with "dry part" and "wet part" =1.000293 -> Refraction = 32.17 minutes using a generally mentioned approximative index of 1.0003 -> Refraction = 33.03 minutes and, last but not least, Bennett's formula -> Refraction = 34.48 minutes At this point it really would be interesting to know to what values Bennett fitted the approxiation formula. Marcel