Answer:
this pattern to occur there must be coherence in the light beams.
you use a flashlight, the rays are incoherent so diffraction patterns cannot occur.
Explanation:
The point pattern that appears in the wall is the result of the interference and diffraction processes through each space of the mesh, for this pattern to occur there must be coherence in the light beams.
The coherence process is that all the rays have the same constant and phase, before the appearance of the lasers, the light is stopped by a small opening and this ray is the one that passes through the slits, with the appearance of the laser this it is consistent from its production process, so opening is not necessary, with this there is much greater intensity and the measurement process is simplified.
When you use a flashlight, the rays are incoherent so diffraction patterns cannot occur.
Polarization has no effect on diffraction patterns so it does not matter if it is vertical or horizontal.