# Homework Help: Plane-polarized electromagnetic waves

1. Aug 7, 2010

### twoscoops

1. The problem statement, all variables and given/known data

Consider a plane-polarised electromagnetic wave in source-free vacuum, with magnetic field B = (1,1,0)BoCos(kz-wt), with Bo = 0.001 Tesla. Find
(1) the direction of propagation of the wave,
(2) average energy density carried by the wave, and
(3) the fraction of this energy which would pass through a perfect polariod sheet at normal incidence, if the angle between B and the transmission axis of the polariser is 30degrees.

2. Relevant equations
Uvpw = eoEo^2cos^2(kz-wt)
Bo=Eo/c
eo = 8.854x10^-12
Not really sure of any others

3. The attempt at a solution
(1)travelling in the +z direction
(2)Bo=Eo/c Eo= 10^5
<Uvpw> = eoEo^2x(average of cos^2) average cos^2 = 0.5
<Uvpw> = 0.5eoEo^2 = 0.398Jm-3

Im pretty sure the first two are right but im stumped how to do part 3.
Thanks

2. Aug 8, 2010

### vela

Staff Emeritus
A polarizer lets the component of the electric field parallel to the transmission axis through; it blocks the perpendicular component.