A homogeneous shaft PQR with fixed supports at both ends is subjected to a torsional moment T at point Q, as shown in the figure. The polar moments of inertia of the portions PQ and QR of the shaft with circular cross-sections are \(J_1\) and \(J_2\), respectively. The torsional moment reactions at the supports P and R are \(T_p\) and \(T_R\), respectively.If \(\frac {T_p}{T_R} = 4\) and \(\frac {J_1}{J_2} = 2\), the ratio of the lengths \(\frac {L_1}{L_2}\) is