The The work function for a metal is 4 ev. to eject the photoelectrons with zero velocity the wavelength of the incident light should be

We know that E or ​Φ = h ν                   Φ is work function of metal
So , h ν = h c / λ 
Thus , λ = h c / ​Φ                  { since h c is nearly equal to 1240 eV - nm } 
=>  λ = ( 1240 eV - nm / 4 eV ) 
=>  λ = 310 nm 
wavelength of incident light is 310 nm or 3100 Å

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