If E ⃗ \vec{E} E and K ⃗ \vec{K} K represent electric field and propagation vectors of the EM waves in vacuum, then magnetic field vector is given by : (ω - angular frequency) :

Question

G1 Physics · Accepted Answer

The magnetic field is perpendicular to both propagation direction and electric field, along K⃗ × E⃗. Also |K⃗| = ω/c and B = E/c. Therefore B⃗ = (1/ω)(K⃗ × E⃗).