Light strikes a long narrow rectangular slit, producing what appears to be a 1-dimensional diffraction pattern oriented horizontally on a screen some distance from the slit. How is the slit oriented?

1. the slit is oriented horizontally
2. the slit is oriented vertically

In general the smaller the dimension of an opening the more spread there is in the light diffracting from the opening. The fact that the diffraction pattern is horizontal means the short side of the rectangular opening is horizontal. The slit must be much longer vertically to give essentially no vertical spread.

If we went to a square opening the diffraction pattern would be two-dimensional.

A circular opening produces a bulls-eye diffraction pattern, a pattern with alternating bright and dark rings.