The wave fronts of a light wave travelling in vacuum are given by x + y + z = c. The angle made by the direction of propagation of light  with the X-axis is

Ray is perpendicular to wave front thus, the vector along ray is same as a vector perpendicular to wave front x+y+z=c. Thus, the vector along the ray is , let ray make  with x-axisIf β andγ are angels between the ray and positive.y-axis and z-axis, respectively, then