2.2 the planes

This figure should be clear enough to explain the definitions of the planes and angles. (the yellow arrow is the incident beam, and the bricks are a grating!)

anyway

`grating plane'

plane parallel to the grating surface

`principle plane'

plane parallel to the x-axis and perpendicular to the grating plane (used for 1D gratings)

`incident plane'

plane perpendicular to the grating plane, containing the incident ray propagating vector.

`alpha'

angle wavevector and z-axis

`delta'

angle wavevector and x-axis