When a surface is irradiated with light of wavelength `4950 Å`, a photocurrent appears which vanishes if a retarding potential greater than 0.6 volt is applied across the phototube. When a second source of light is used, it is found that the critical potential is changed to 1.1 volt.
The work- function of the emitting surface is i

Explanation

In first case the energy of incident photon in eV is
`E_(1) = ( 12431)/( 4950) eV = 2.51eV`
The maximum kinetic energy of ejected electrons is
`KE_("max1") = eV_(01) = 0.6 eV`
Thus work function of metal surface is given as
`phi = E_(1) - KE_("max1") = 2.51 - 0.6 eV = 1.91 eV`
In second case the maximum kinetic energy of ejected electrons will becoms
`KE_("max2") = eV_(02) = 1.1 eV`
Thus the incident energy of photons can be given as
`E_(2) = phi+ KE_("max2")`
`E_(2) = 1.91 +1.1 eV = 3.01 eV`
Thus the wavelength of incident photons in second case will be
`lambda= ( 12431 )/( 3.01 )Å = 4129.9Å`
When magnetic field is present there will be no effect on the stopping potential as magnetic force can not change the kinetic energy of ejected electrons.