How do PAPI's and VASI's work?

[shon7]

How do PAPI's and VASI's work? One response I got for (VASI) is that they are divided and thus from a higher altitude you can only see white and lower you see red? But how would this work in the case of a PAPI with 4 separate lights?

Thanks for your answers. Sorry if the question is somewhat ambigious.