@staticmethod
def _cauchy_equation(wavelength, coefficients):
    """
        Helpful function to evaluate Cauchy equations.

        Args:
            wavelength (float, list, None): The wavelength(s) the
                Cauchy equation will be evaluated at.
            coefficients (list): A list of the coefficients of
                the Cauchy equation.

        Returns:
            float, list: The refractive index at the target wavelength(s).
        """
    n = 0.0
    for i, c in enumerate(coefficients):
        exponent = 2 * i
        n += c / wavelength ** exponent
    return n
