The direction of propagation of light is perpendicular to the wave front and is symmetric about x,y and z axis.
∴ Angle made by the light with x,y&z axis is same.
∴cosα=cosβ=cosγ(α,β&γ are angle made by light with x,y&z axis respectively)
Also cos2α+cos2β+cos2γ=1 [Sum of direction cosines]
∴α=cos−131