;+
; NAME: 
;  refractivity
; PURPOSE: 
;  return the refractivity, give the number density of constituents
; DESCRIPTION:
;  Using Allen's Astrophysical Quantities (AQ) 4th Ed.
;  Arther Cox, editor, p. 100
; CATEGORY:
;  Occultation analysis
; CALLING SEQUENCE:
;  nu = refractivity(lam, n_array)
;
; INPUTS:
;  lam - wavelength in micron
;  n_array = list of number densities for
; H2, He, Ar, N2, CH4, CO, O2, CO2, NH3
;
; OPTIONAL INPUT PARAMETERS:
; KEYWORD INPUT PARAMETERS:
; OUTPUTS:
;  nu   - refractivity = n-1 (unitless)
;  
; KEYWORD OUTPUT PARAMETERS:
; COMMON BLOCKS:
; SIDE EFFECTS:
; RESTRICTIONS:
; PROCEDURE:
; MODIFICATION HISTORY:
;  01/11/26 - Written by Leslie A. Young, Southwest Research Institute
;-

function refractivity, lam, n

  losc = 2.6868e19  ; Loschmidt's number
  r = n/losc ; ratio w/ r. to losc

  refr = $
    r[0] * nuSTP_H2(lam) + $
    r[1] * nuSTP_He(lam) + $
    r[2] * nuSTP_Ar(lam) + $
    r[3] * nuSTP_N2(lam) + $
    r[4] * nuSTP_CH4(lam) + $
    r[5] * nuSTP_CO(lam) + $
    r[6] * nuSTP_O2(lam) + $
    r[7] * nuSTP_CO2(lam) + $
    r[8] * nuSTP_NH3(lam) 
    
  return, refr
end