A plane monochromatic electromagnetic wave with wavelength $=4\mathrm{.}3$ cm$,$ propagates through a vacuum. Its magnetic field is described by

B$=($Bxi$^+$Byj$^)$cos$($kz$+$t$)$

where Bx $=2\mathrm{.}3$ X $10-6$ T$,$ By $=3\mathrm{.}8$ X $10-6$ T$,$ and i$-$hat and j$-$hat are the unit vectors in the $+$x and $+$y directions, respectively.