Light falls on a photoelectric surface that has a work function of 2.90 eV. If the voltage required to stop the ejected electrons is 0.210 V, what is the wavelength of the incident radiation?
The energy provided by a photon is
e*(Work function) + e*(stopping voltage)
= (1.602*10^-19 C)*(3.11 J/C)
= 4.98*10^-19 joules
Set that equal to h*c/(wavelength) and solve for wavelength.
Wavelength = 6.62*10^-34*3.00*10^8/4.98*10^-19
= 3.99*10^-7 m = 399 nanometers (violet)