The work function for the following metals is given:

Na: 2.75 eV; K: 2.30 eV; Mo: 4.17 eV; Ni: 5.15 eV. Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 Å from a He-Cd laser placed 1 m away from the photocell? What happens if the laser is brought nearer and placed 50 cm away?

Given:


Element



Work function Φ0



Na



2.75 eV



K



2.30eV



Mo



4.17 eV



Ni



5.15 eV



Wavelength of radiation from laser = 3300 Å = 3300 × 10-10m


Energy radiated by the laser, E is given by,


…(1)


Where,


E = Energy of photon


h = Planck’s constant = 6.6 × 10-34 Js


c = 3 × 108 ms-1


λ = wavelength of photon


putting values in equation (1) we get,



E = 6 × 10-19 J


E = (6/1.6) × 10-19


E = 3.158 eV


We observe that Na and K have work functions less than the energy radiated by the laser so photoelectric effect will take place in case of them, whereas no photo-electric effect will occur in case of Mo and Ni.


Even if the laser is brought closer, it will have no effect on the outcome as the photo-electric effect is independent of the intensity of light.


29