1. The problem statement, all variables and given/known data In laboratory's frame the angle between vectors E and H is given φ. Find the frame where these vectors will be parallel. 2. Relevant equations 3. The attempt at a solution I suppose I should require equal angle between each of the E' and H' and axes in the ''new'' frame. Means this: E'||/|E'|=H'||/|H'| After this I lose the key. How can I implement the given angle φ?