The threshold wavelength of a metal is 5800Ao. The metal surface is illuminated with a light of wavelengthof 2800Ao and the electron is emitted with a certain energy. Calculate the potential required to bring the electron to rest.

Energy associated with a light of wavelength 2800 A°=hv=hcλor E=6.626×10−34Js3×108ms−12800×10−10m=7.099×10−19JThreshold Energy,E0=hcλ0=6.626×10−34Js3×108ms−15800×10−10m=3.427×10−19J K.E. =12mv2=E−E0=7.099×10−19J−3.427×10−19J=3.672×10−19J=3.672×10−19J1.602×10−19J/eV=2.292eVEnergy required to stop the electron = 2.292 eV∴ Potential required to bring the electron to rest = 2.292 eV.