A metal surface is illuminated by a radiation of wavelength 4500 Å The ejected photo-electron enters a constant magnetic field of \( 2 \, \text{mT} \) making an angle of \( 90^{\circ} \) with the magnetic field. If it starts revolving in a circular path of radius \( 2 \, \text{mm} \), the work function of the metal is approximately:
