Question

Why ordinary plane transmission gratings cannot be used to produce diffraction effects in x rays?

Answer 1

I believe that it is because the slits can act as a polarising filter as they are so narrow and the experiment assumes you are using plane polarised light. So if you're diffraction grating was not normal to the light then the intensity would be less or even mom according to Malu's Law so the intensity would be directly proportional to Cos^2 (angle of source from normal).