A monochromatic light is incident on a metallic plate having work function \( \phi \). An electron, emitted normally to the plate from a point A 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 \( m \), \( h \) is Planck's constant and \( c \) is velocity of light. Take the magnetic field exists throughout the path of electron)
