According to the sources, in an ideal transformer, the ratio of the secondary voltage ($V_s$) to the primary voltage ($V_p$) is equal to the ratio of the number of turns in the secondary coil ($N_s$) to the number of turns in the primary coil ($N_p$), expressed as $V_s/V_p = N_s/N_p$ . Given the turns ratio $N_p/N_s = 1/2$, it follows that $N_s/N_p = 2/1$. Therefore, the ratio $V_s:V_p$ is 2:1.