The work function of a substance is 4 eV. What is the approximate longest wavelength of light that can cause photo-emission ?

309 nm 209 nm 109 nm 9 nm

309 nm

$λ_{th}=\frac{hc}{\phi}=\frac{12400}{4}Å=3100Å=310nm$ $λ≤λ_{th}⇒λ≤310nm$