In introductory college chemistry, bonds between elements that have an electronegativity difference $\Delta EN > 0.4$ are generally consider to be polar (and thus have a dipole moment), while $\Delta EN < 0.4$ bonds are considered to be nonpolar. I think more advanced chemistry classes generally don't adhere strictly to this threshold, but I'm still curious if this 0.4 threshold has any meaning (or if it's just arbitrary).
For example, if there's an experiment plotting $\Delta EN$ vs. the intermolecular force strength, and there's a big jump on the graph at 0.4, that would be convincing evidence. Does anyone have an explanation along this line of reasoning?
(My question could also apply to the $\Delta EN > 1.7$ threshold for ionic vs. polar covalent. I saw a question on this before here, but I think it's worth further discussion.)