def vacuum_to_air(lambda_vac):
    """
    Vacuum-to-air wavelength conversion

    Args:
        lambda_vac: vacuum wavelength in Angstrom

    Returns:
        lambda_air: air wavelength in Angstrom

    Source:
        http://www.astro.uu.se/valdwiki/Air-to-vacuum%20conversion

    Original reference:
        Donald Morton (2000, ApJ. Suppl., 130, 403)

    Example:

    >>> # The following code converts wavenumber=2000 cm**-1 to wavelength in Angstrom
    >>> vacuum_to_air(1e8/2000)
    49986.36934549974
    """
    s = 10000.0 / lambda_vac
    n = 1 + 8.34254e-05 + 0.02406147 / (130 - s ** 2) + 0.00015998 / (38.9 -
        s ** 2)
    lambda_air = lambda_vac / n
    return lambda_air
