Physics 208, Spring 08,Quiz Week 13Apr. 14, 20081. Short problem: A red and blue beam of light have a power of 3 mW and wavelengths respectively of 750 nm and 430 nm. How many photons/second do they produce? [Hint: hc = 1240 eV x nm and 1 eV = 1.6 x 10-19J.] 2. What is the maximum velocity of a photoelectron emitted from a surface whose work function is 5.0 eV when illuminated by a light whose wavelength is 200 nm? Solution: Power = energy/time = Number of photons x hf/time The blue light produces Number of photons/time = Power/hf = Power x lambda/hc = 3 mW x 750 nm/(1240 eV x nm) = 3 x 10-3eV x 750 nm/(1.6 x 10-19s x 1240 eV x nm) = 1.13 x 1016photons/s and the red light produces 6.50 x 1015photons/s. Red light has less energy per photon (because it has lower frequency) so it needs more photons than the blue light to produce the same power. Multiple choice