As per the definition of direction cosines, the cosine values of the angles formed by the directed line with the x-axis, y-axis and z-axis.

Here we consider the directed line to be the y-axis.

So from the below figure, we can say,

α = the angle formed by the y-axis with x-axis = 90°

β = the angle formed by the y-axis with y-axis = 0°

γ = the angle formed by the y-axis with y-axis = 90°

Therefore,

cos α = cos 90° = 0

cos β = cos 0° = 1

cos γ = cos 90° = 0

Hence the direction cosines of y-axis are 0, 1, 0.