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

Photoelectric emission will take place
if the energy of incident photon is greater than the
work function of metal on which photon is falling
i.e., `(hc)/lambda≥phi_0`
Here, `(hc)/lambda=((6.63xx10^(-34))xx(3xx10^8))/(3300xx10^(-10))`
`6xx10^(-19)J`
`(6xx10^(-19))/(1.6xx10^(-19))eV=3.75eV`
As work function of `M_0 and N_i` is greater than
the energy of incident photon, hence no photoelectric
emission will take place from `M_0 and N_i`. The photoelectric emission from `N_a` and K will increase
in proportion to intensity.