Practice question

Scanning electron microscopy is optimized for surface morphology rather than internal ultrastructure. A finely focused electron beam sweeps across specimen surface, generating secondary electrons emitted from top few nanometers and backscattered electrons dependent on composition. Detectors collect these signals, modulating image brightness according to topography, edge effects and material contrast. Heavy-metal coated biological samples thus reveal exquisite details of pollen exine sculpture, bacterial biofilms, insect cuticle hairs and fractured tissue surfaces with large depth of field, producing intuitive three-dimensional appearance impossible with transmitted light or TEM thin sections.