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.