def v_theta(self):
        """
        Return the ``(v, theta)`` equivalent of the (normalized) quaternion.

        Returns
        -------
        v : float
        theta : float

        """
        # compute theta
        norm = np.sqrt(np.sum(self.wxyz**2))
        theta = 2 * np.arccos(self.wxyz[0] / norm)

        # compute the unit vector
        v = np.array(self.wxyz[1:])
        v = v / np.sqrt(np.sum(v**2))

        return v, theta
Return the ``(v, theta)`` equivalent of the (normalized) quaternion.

        Returns
        -------
        v : float
        theta : float