When electrons from the beam impact the surface of a solid, some of them may be reflected (as "backscattered" electrons), while others penetrate the surface, where they collide with the particles of the solid. In non-elastic collisions they lose their kinetic energy. It has been proved, both theoretically and experimentally, that they can "travel" only a very small distance below the surface before they transfer all their kinetic energy into heat. This distance is proportional to their initial energy and inversely proportional to the density of the solid. Under conditions usual in welding practice the "travel distance" is on the order of hundredths of a millimeter. Just this fact enables, under certain conditions, fast beam penetration.