The light wave is propagating with plane wave fronts described by the equation constant. The wave propagates in a direction perpendicular to these wave fronts. This direction is symmetric with respect to the , and axes.

Since the wave is symmetric about these axes, the angle it makes with each axis is the same. Let these angles be , and , which are the angles made by the light with the , and axes, respectively.

This implies:

According to the sum of the squares of the direction cosines:

Substituting the equality of direction cosines, we get:

Thus, the angle is given by: