def get_taus(cos_theta, n2n1=<NUM_LIT>):
return <NUM_LIT>/(<NUM_LIT:1>+csqrt(<NUM_LIT:1>+(n2n1**<NUM_LIT:2>-<NUM_LIT:1>)*cos_theta**-<NUM_LIT:2>))<EOL>
Calculates the Fresnel reflectivity for s-polarized light incident on an
interface with index ration n2n1.

Parameters
----------
    cos_theta : Float or numpy.ndarray
        The _cosine_ of the angle of the incoming light. Float.
    n2n1 : Float, optional
        The ratio n2/n1 of the 2nd material's index n2 to the first's n1
        Default is 0.95

Returns
-------
    Float or numpy.ndarray
        The reflectivity, in the same type (ndarray or Float) and
        shape as cos_theta