def rho2_rho1(M1, gamma):
    """Density ratio across a normal shock (eq. 3.53)

    :param <float> M1: Mach # before the shock
    :param <float> gamma: Specific heat ratio

    :return <float> Density ratio r2/r1
    """

    n1 = (gamma + 1.0) * M1 ** 2
    d1 = 2.0 + (gamma - 1.0) * M1 ** 2

    return n1 / d1