Answer :
The three directions are all "orthogonal". That means that the direction of the
wave is forward, the direction of the electrostatic field is across it ... say up
and down ... and the magnetic field is also across it but perpendicular to the
electrostatic field ... left and right.
Saying it another way . . .
The electrostatic field and the magnetic field are perpendicular to each other, and
both of them are perpendicular to the direction in which the wave is traveling.
