If it hits a buried object or a boundary with different electrical properties, the receiving antenna records variations in the reflected return signal.
See also - http://www.soe.ucsc.edu/~milanfar/GPR2002SPIE.pdf [Broken] (pdf file).It is the only reliable method for detecting buried plastic containers.
The paint is important in a stealth aircraft, but even more important is the form.abercrombiems02 said:Sure it can, radar doesn't act like a metal detector. If a surface can reflect light, radar can sure spot it. When you look at the plastic top on a bottle of soda or water you couldn't see it if it didnt reflect light. Just about anything that is opaque can be seen by radar, unless it is stealth. The way stealth aircraft work is that their covered in RAM or radar absorbant material. This is kind of like a clear paint whose media has a very high index of refraction. When light enters this "clear" layer of paint it becomes trapped inside and dissipates into thermal energy.