Let $\vec { a } , \vec { b }$ and $\vec { c }$ be three unit vectors, out of which vectors $\vec { b }$ and $\vec { c }$ are non-parallel. If $\alpha$ and $\beta$ are the angles which vector $\vec { a }$ makes with vectors $\vec { b }$ and $\vec { c }$ respectively and $\vec { a } \times ( \vec { b } \times \vec { c } ) = \frac { 1 } { 2 } \vec { b }$, then $| \alpha - \beta |$ is equal to :\\
(1) $90 ^ { \circ }$\\
(2) $60 ^ { \circ }$\\
(3) $45 ^ { \circ }$\\
(4) $30 ^ { \circ }$