All electrons ejected from a surface by incident light of wavelength 200nm can be stopped before travelling 1m in the direction of uniform electric field of 4 N/C. The work function of the surface is

The electron ejected with maximum speed vmax are stopped by electric  field E=4 N/C after travelling a distance d=1 m12mvmax2=eEd=4eV The energy of incident photon =1240200=6.2eV From equation of photo electric effect 12mvmax2=hv−ϕ0∴ϕ0=6.2−4=2.2eV