def refractive_index(wavelength, a=1.5375, b=0.00829045, c=-0.000211046):
    """Cauchy's equation - dispersion formula
    Default coefficients are for NOA61.
    https://refractiveindex.info/?shelf=other&book=Optical_adhesives&page=Norland_NOA61
    """
    return a + b / (wavelength * 1e6) ** 2 + c / (wavelength * 1e6) ** 4