The observed polarization properties of an extended polarized radio source situated behind an irregular foreground Faraday screen are studied. Depolarization laws are derived for both a uniform and a linear source. The polarization behaviour is not strongly dependent on the assumed power spectrum of rotation measure (KM) fluctuations and generally behaves as a power law of wavelength when the root mean square rotated angle exceeds unity, a much slower fall off than given by Burn's formula. Polarization fluctuations are large, and the observation of patchy depolarization does not necessarily imply a clumpy foreground screen. The observed width of the KM distribution does depend on the assumed KM power spectrum. Internal and foreground depolarization have a similar wavelength dependence, and separating them will be difficult. Deductions of the screen properties from a Burn-type law are likely to be false. I estimate or constrain the properties of the depolarization screens in the Coma galaxy cluster and about quasars.