Which correctly describes secondary electron and backscattered electron images in SEM?

Answer: A. Secondary electrons mainly show surface topography, and backscattered electrons show compositional (atomic number) differences

Secondary electrons are low-energy electrons of 50 eV or less and can escape only from depths of a few nanometers below the surface. They are therefore strongly affected by surface tilt, topography and edges, producing images of shape with a three-dimensional look. Backscattered electrons are incident electrons scattered back out by the atoms of the specimen; they have high energy and come from deeper regions. The yield of backscattered electrons increases with atomic number, giving compositional images in which regions of heavy elements appear bright. The choice in which the secondary electron image shows atomic number and the backscattered electron image shows topography reverses the roles of the two, and saying their escape depths are the same is also wrong. In failure analysis, the two images are compared to look for foreign particles and precipitates.