Planck's constant, \(h = 4.1357 \times 10^{-15}eVs\), but \(1eV = 1.60218 \times 10^{-19}J\)
\(h = 6.63 \times 10^{-34}Js\)
\(hf = \phi + E_k\)
\(f\) = the frequency of the incident light in hertz (Hz)
\(\phi\) = the work function in joules (J)
\(E_k\) = the maximum kinetic energy of the emitted electrons in joules (J)
Given wavelength, \(\lambda\) = 1200\(\unicode{xC5}\), we can find frequency thus: \(f = \cfrac{c}{\lambda}\)
where speed of light, \(c = 3.0 \times 10^8\)m/s
\(\phi = hf - E_k\)
\(hf = \cfrac{3.0 \times 10^8}{1200 \times 10^{-10}} \times 6.63 \times 10^{-34}\)
\(hf = 1.6575 \times 10^{-18}J\)
\(E_k = 4eV = 4 \times 1.60218 \times 10^{-19}J\)
\(= 6.4087 \times 10^{-19}J\)
\(\phi = (1.6575 \times 10^{-18}J) - (6.4087 \times 10^{-19}J)\)
\(\phi = 1.01663 \times 10^{-18}J\)
\(\phi = 1.01663 \times 10^{-18}J \times \cfrac{1eV}{1.60218 \times 10^{-19}J}\)
\(\phi = 6.36\)eV