Adam asked:Why does the B-field not get cancelled out during polarisation?

I supposed you are trying to ask about polarisation of EM waves--- which is basically an oscillating B and E field that are mutally perpendicular to each other. If the polarising axis is parallel to the E-field, then the E-field gets out without any attenuation. So the E-field will oscillate with the same amplitude. Any E-field that is present will always be accompanied by the B-field which is always perpendicular to it, so the B-field will also remain unattenuated. If the polarising axis is at an angle to the B-field, then we have the situation of the E-field being rotated by the same angle and the magnitude will just be the component parallel to the E-field. Once E-field is reduced, the perpendicular B-field (which has also rotated to be always perpendicular to the E-field) will also have a smaller amplitude.

Will give u more info another day when i have more time to write....