The deflecting devices come in pairs and surround the picture tube. The left and right devices move the electron beam horizontally, and the top and bottom devices move the electron beam up and down. The screen is coated inside with a fluorescent material that lights up when it is hit with the electron beam and it stays lit for some time after the beam leaves it. By controlling the deflection of the dot, it can be displayed anywhere on the screen. But how does that dot become a picture?