The resolution of microscopes is limited by the fact that two points can only be perceived as separate if they are further apart than half the wavelength of the radiation used for observation. The wavelength of electron beams is much smaller than that of visible light. Therefore, much smaller structures can be analysed. SEMs observe the reflected radiation. The electron beam scans the object point by point. A very special advantage of the SEM is the depth of focus it can capture, so that it can provide sharp images of three-dimensional structures of great depth. There are also transmission electron microscopes (TEM), which are less important because they can only be used for very thin objects that are transparent to electron beams.