Isosceles triangles $A B C(A B=B C)$ and $A_{1} B_{1} C_{1} \quad\left(A_{1} B_{1}=B_{1} C_{1}\right)$ are similar and $A C: A_{1} C_{1}=5: \sqrt{3}$.

Vertices $A_{1}$ and $B_{1}$ are located on sides $A C$ and $B C$, respectively, and vertex $C_{1}$ is on the extension of side

$A B$ beyond point $B$, and $A_{1} B_{1} \perp B C$. Find the angle $B$.