def relative_refractive_index(n_core, n_clad):
    """
    Calculate the relative refractive index (Delta) for an optical fiber.

    Args:
        n_core :  the index of refraction of the fiber core      [-]
        n_clad:  the index of refraction of the fiber cladding   [-]

    Returns:
        the relative refractive index (Delta)                    [-]
    """
    return (n_core**2 - n_clad**2) / (2 * n_core**2)