
A slope away from the radar illumination with an angle that is steeper than the sensor depression angle provokes radar shadows.
It should be also noted that the radar shadows of two objects of the same height are longer in the far range than in the near range.
Shadow regions appear as dark (zero signal) with any changes due solely to system noise, sidelobes, and other effects normally of small importance.
Let us consider the mountain as sketched in the animation. Points A, B, C and D are defining different parts of the target when vertically projected on the ground (the orthogonal projection, as it is done in conventional cartography). However, the segment between B and C is not giving any contribution in the slant range direction which is the SAR projection ( B''  C'' ), due to the geometry of the mountain.
Note also the distance AB becoming orthogonal projected A' B'. In the slantrange projection this is A'' B'', much shorter, due to the foreshortening effect. 