The magnetic field vec$(B)(z,t)$ of a plane monochromatic electromagnetic wave of wavelength $=681nm$ that is propagating in a vacuum in the $z$ direction is described by:

vec$(B)(z,t)=B_{1}sin(kz-t)(hat()+$hat$())$

where $B_1=9\mathrm{.}00{10}^{-6}T,$ and hat$()$ and hat$()$ are the unit vectors in the positive$-x$ and positive$-y$ directions, respectively.