Scanning electron microscope. Researcher using a scanning electron microscope (SEM, upper left) to study the head of an ant. SEMs use an electron beam to obtain three-dimensional images of objects. The samples must be dehydrated so that they are not damaged by the vacuum in the microscope. They are also coated in gold to allow them to conduct the beam current away. The electron beam is then swept over the sample, which causes secondary electrons to be emitted. It is these electrons which are used to form the image. SEMs can provide magnifications of up to around 300,000 times.