Final answer:
The focusing cup is negatively charged to confine the electron beam to a tiny area on the anode, necessary for creating precise imagery.
Step-by-step explanation:
The focusing cup in an electron beam apparatus is negatively charged so that it electrostatically confines the electron beam to a small area of the anode. This is essential for creating precise and accurate images, as in the case of X-ray tubes or cathode-ray tubes (CRTs). Similarly, when discussing inkjet printers, the ink droplets become charged by capturing electrons, allowing them to be directed precisely onto the paper. It's important to understand the behavior of charged particles under the influence of electric fields since they behave differently based on their charges. For instance, alpha particles are deflected less due to their positive charge and larger mass compared to the more deflected beta particles, which are negatively charged and lighter. Gamma rays, being uncharged, are not affected by electric fields.