The governing equation from The Photoelectric Effect is
KE = h f - W and c = f L ==> KE = h c / L - W
where L = Lambda = wavelength, KE is energy of the emitted photon, h = Plack's constant
and f is the frequency of the input light, c = speed of light/EM energy
KE = h c / L - W
At threshold KE = 0
So h c / L = W
L = h c / W
L = 4.1e-15 ev-sec 3e8 meters/sec / 2ev
L = 6.15e-7 meters
L = 6.15e-7 m ( e9 nanometers / meter ) = 615 nm
So the wavelength is 615 nanometers
