In scanning electron microscopy the sample being imaged is bombarded with electrons from an electron gun. The electron beam is refined and directed at the sample using a series of electromagnets. When the beam reaches the sample, two different types of electrons can be observed to form the final image: secondary electrons and back scatter electrons. The secondary electrons are those which are released by the atoms in response to the electron beam. The number of secondary electrons detected is a function of the angle between the sample and the beam. These electrons are found very close to the surface and give an indication of the surface topography of the sample. The second type of electron that can be detected are called back scatter electrons. These electrons are the same ones from the electron beam reflected back onto a detector. The electrons from the beam travel farther into the sample than secondary electrons and thus are a better representation of surface composition. SEM can provide a resolution of better than one nanometer.