A monochromatic light is incident on a metallic plate having work

function . An electron, emitted normally to the plate from a point with maximum kinetic energy, enters a constant magnetic field, perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point B . The distance between A and B is :
(Given : The magnitude of charge of an electron is e and mass is is Planck's constant and c is velocity of light. Take the magnetic field exists throughout the path of electron)