Single slit diffraction pattern in 2D

[alingy1]

Hi all,

I was looking at previous exams and I saw a question with single slit diffraction. Please look at picture.

So, this made me think: "Wow, I never thought single slit diffraction could be applied in 2D with one pattern horizontal and the other vertical."

Then, I thought why is there no diffraction patterns along all the surface of the viewing screen. Can anyone explain why? The textbook I am using emphasizes how when a wave enters a boundary place, each point sends out its own spherical wavelet in 3D. So, why don't we see that on the viewing screen?

 

[Bob_Dole]

Constructive and destructive interference I presume. You can draw it, we did this in first year undergrad.