Let $\vec{a}=a_i \hat{\imath}+a_2 \hat{\jmath}+a_3 \hat{k}$ and $\vec{b}=b_1 \hat{\imath}+b_2 \hat{\jmath}+b_3 \hat{k}$ be two vectors such that $|\vec{a}|=1 ; \vec{a} \cdot \vec{b}=2$ and $|\vec{b}|=4$. If $\vec{c}=2(\vec{a} \times \vec{b})-3 \vec{b}$, then the angle between $\vec{b}$ and $\vec{c}$ is equal to :