Dust can generate polarized microwaves in two ways. First, shown left, microwaves passing through a dust cloud can become polarized if the dust in the cloud is aligned with the Milky Way's own magnetic field. Only those microwaves polarized along the direction of the magnetic field can pass through the dust, which acts as a filter. Or second, shown right: dust heated by distant starlight emits microwaves of its own, which are polarized perpendicular to any magnetic field aligning that dust.