25．在长方体$ABCD-A_{1}B_{1}C_{1}D_{1}$中，$DD_{1}=2AB=2BC=2$，动点$P$满足$\frac{{D_1}P}{{D_1}B}=\lambda ({0<\lambda <1})$且在线段$BD_{1}$上，当$AP$与$CP$垂直时，$\lambda$的值为 ___$\frac{2}{3}$___．