Photons and Photoelectric Effect



  1. A detached retina is being "welded" back by using 20 ms pulses from a 0.5 Watt laser operating at a wavelength of 643 nm. How many photons are in each pulse ?

    2. 0.5 watts = 0.5 J/s = 0.5 x 20 x 10-3 J/pulse = 0.01 J/pulse

    Energy of photons = hf = hc/(lambda) = (6.6 x 10-34 x 3 x 108)/643 x 10-9 = 3 x 10-19 J

    Number of photons per pulse = 0.01/(3 x 10-19) = 3.3 x 1018



  2. The threshold frequency for photoelectric emission in calcium is 7.7 x 1014 Hz. Find the maximum energy in electron-volts of the photo-electrons when light of frequency 1.2 x 1015 Hz is directed on a calcium surface.

    3. hfc = Work function

    hf = Work function + KE

    KE = hf - Work function = hf - hfc = h (f - fc) = 6.6 x 10-34 x (12 - 7.7) x 1014 = 2.84 x 10-19 J

    KE in eV = 2.84 x 10-19/1.6 x 10-19 = 1.77 eV