The work function for the following metals is given Na: 2.75 eV, K:2.30 eV , Mo: 4.17 eV , Ni: 515 eV.
Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 A from a `He-Cd` laser placed 1m away from the photocell? What happens if the laser is brought nearer and placed 50 cm away?

Explanation

Energy of the incident radiation of wavelength `lambda` is, `E=(hc)/lambda=((6.63xx10^(-34))xx(3xx10^(8)))/(3300xx10^(-10)xx(1.6xx10^(-19)))=3.75 eV`
This energy of the incident radiation is greater than the work function of Na and K but less than those of MO and Ni, so photoelectric emission will occur only in Na and K metals and not in MO and Ni. If the laser is brought closer, the intensity of incident radiation increases. This does not affect the result regarding MO and Ni metals, which photo electric emission from Na and K will increases in proportion to intensity.