Determine the velocity of a reference frame S

[Hydeyuki]

Can someone give me a clue/hint to finish this question?

"In a reference frame S, the electric and magnetic fields of an electromagnetic wave E
and B are perpendicular to each other. Determine the velocity of a reference frame S'
in which (a) E' = 0 and (b) B' = 0"

 

[vela]

How do the electric and magnetic fields transform under a Lorentz boost?

 

[Markus Hanke]

Yes, as vela said above - write down the Lorentz transformation rules for both E and B. You will see immediately what needs to happen to get E'=0 and B'=0...