The angle between 2 vectors $\vec{a}$ and $\vec{b}$ with magnitude 1 and 2 respectively when $\vec{a} . \vec{b}=1$ is:

$\frac{\pi}{6}$ $\frac{\pi}{3}$ $\frac{\pi}{2}$ $\frac{\pi}{4}$

$\frac{\pi}{3}$

The correct answer is Option (2) → $\frac{\pi}{3}$