A light wave is propagating with plane wave fronts of the type \( x+y+z=\text{constant} \). The angle made by the direction of wave propagation with the x-axis is:
\( \cos^{-1}\!\left(\frac{1}{3}\right) \)
\( \cos^{-1}\!\left(\sqrt{\frac{2}{3}}\right) \)
\( \cos^{-1}\!\left(\frac{2}{3}\right) \)
\( \cos^{-1}\!\left(\frac{1}{\sqrt{3}}\right) \)
