Question:

For plane electromagnetic waves propagating in the z direction, which one of the following combinations gives the correct possible direction for E and B fields respectively?

(→i+→j)and(3→i+→j)

(→i+2→j)and(2→i-→j)

(2→i+3→j)and(→i+2→j)

(3→i+4→j)and(4→i-→j)

The →E and →B are ⊥ to each other, their dot product must be zero →E.→B=0
Checking option A (→i+2→j) . (2→i-→j) = (1)(2) + (2)(-1) = 2-2 = 0