Question

The work function of 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 a photoelectric emission for a radiation of wavelength 3300 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 ?

Answer 1

Energy of incident photon, E = hc/λ

Here λ= 3300 = 3300 × 10–10 m = 3.3 × 10–7 m

Photoelectric emission is only possible if energy of incident photon is equal to or greater than the work function. For Na and K this condition is satisfied, hence photoelectric emission is possible; but in the case of Mo and Ni, the energy of incident photon is less than the work function ; hence photoelectric emission is not possible.

If source is brought nearer, then the intensity of incident radiation increases but frequency of a photon *** the same ; therefore Mo and Ni will still not show photoelectric effect ; however in the case of Na and K the current will increase in same proportion as the increase in intensity takes place