Electrons in the beam impact the surface of the sample with a range of energies. A variety of processes occur to these electrons. The principle means by which we observe the surface with our Hitachi TM3030 is by those electrons which are scattered back from the surface, nearly 180 degrees, these are called Backscattered Electrons.
Backscattering is an elastic process. The electrons are more likely to be backscattered by heavier elements. This means that we have elemental contrast in our image as well as topographical contrast, and the elemental contrast varies, according to atomic number, as shown in the graph below.