electron optics: the branch of electronics that deals with beams of electrons and their focusing and deflection by magnetic fields
natural science: the sciences involved in the study of the physical world and its phenomena
tropic: either of two parallels of latitude about 23.5 degrees to the north and south of the equator representing the points farthest north and south at which the sun can shine directly overhead and constituting the boundaries of the Torrid Zone or tropics