Sometimes beam electrons interact with the electrons present in the atom rather than the nucleus. Since all electrons are negatively charged, the beam electrons will repel the electrons present in the sample. This interaction causes the beam electrons to slow down as it repels the specimen electrons, The repulsion may be so great that the specimen electrons are pushed out of the atom, and exit the surface of the sample, these are called secondary electrons. Unlike the BSE, the secondary electrons are moving very slowly when they leave the sample. Since they are moving so slowly, and are negatively charged, they can be attracted to a detector which has a positive charge on it. This attraction force allows you to pull in electrons from a wide area and from around corners in much the same way that a vacuum pulls in dust particles. The ability to pull in electrons from around corners is what gives secondary electron images a 3-dimensional look.