What is Scanning electron microscope?
[[File:Misc pollen.jpg|thumb|These [[pollen grain]]s taken on an SEM show the characteristic [[depth of field]] of SEM [[micrograph]]s.]] The '''scanning electron microscope''' (SEM) is a type of [[electron microscope]] that uses a focused [[beam]] of high-energy electrons in producing a variety of signals at the surface of a [[solid]] specimen. The signals produced by the interacting electrons contain useful [[information]] such as the shape, atomic structure and conductivity. ==Introduction== When an electron hits the surface, it may be reflected (''backscattered''), absorbed, or conducted away. Electrons that are absorbed can cause the atom that they hit to become unstable, forcing it to give off another electron (a secondary electron), or to give off light in order to stabilize. Different detectors, for the different types of reactions, may be fitted to an electron microscope, depending on what is being looked for. The magnification that can be achieved in a scanning electron microscope depends on how narrow the beam of electrons that strikes the surface can be, and can reach 1 [[nanometer]], about the size of 3 to 5 [[atom]]s. The control of the beam is achieved using magnetic fields, with other magnetic fields being used to shape the beam, and to move it across the sample. The range of magnification may range from 30x to as high as 500,000x. ==Sample== The electron beam is affected by air and water molecules, so the sample must be placed in a [[vacuum]]. The sample must also be conductive to allow the electrons not reflected or absorbed to be conducted away. This gives some limits on the type of sample that can be used. In the case of samples from [[plant]]s or [[animal]]s, it often means that they must first be coated in gold before they can be placed in the microscope. This is done in order to preserve the sample and keep it from changing or decaying throughout the scanning process. ==Components== The scanning electron microscope is made up of several components: 1. The electron gun is located at the ve