The Williams tube depends on an effect called secondary emission that occurs on cathode ray tubes (CRTs). When the electron beam strikes the phosphor that forms the display surface, it normally causes it to light up; however, if the beam energy is above a given threshold (depending on the phosphor mix) it also causes electrons to be struck out of the phosphor. These electrons travel a short distance before being attracted back to the CRT surface and falling on it a short distance away.