A ray of light travelling in the direction  $\frac{1}{2}(\hat{\mathrm{i}}+\sqrt{3}\hat{\mathrm{j}})$  is incident on a plane mirror. After reflection, it travels along the direction  $\frac{1}{2}(\hat{\mathrm{i}}-\sqrt{3}\hat{\mathrm{j}})$. The angle of incidence is