In electromagnetic duality, where does this relation come from?
$$\overrightarrow{E} \rightarrow \cos \alpha\overrightarrow{E} - \sin \alpha\overrightarrow{ B}$$ $$\overrightarrow{B} \rightarrow \cos \alpha\overrightarrow{B} + \sin \alpha\overrightarrow{ E}$$ I was familiar with $E \rightarrow B$ and $B \rightarrow -B$ but not with the one above.
