Unpolarized light of intensity I passes through an ideal polariser...

Unpolarized light of intensity $I$ passes through an ideal polariser $A$. Another identical polariser $B$ is placed behind $A$. The intensity of light beyond B is found to be $\dfrac{I}{2}$. Now another identical polariser $C$ is placed between $A$ and $B$. The intensity beyond B is now found to be $\dfrac{I}{8}$ . The angle between polariser $A$ and $C$ is