Image intensifiers used in night-vision devices create a bright image from dim l
ID: 1768313 • Letter: I
Question
Image intensifiers used in night-vision devices create a bright image from dim light by letting the light first fall on a photocathode. Electrons emitted by the photoelectric effect are accelerated and then strike a phosphorescent screen, causing it to glow more brightly than the original scene. Recent devices are sensitive to wavelengths as long as 900 nm, in the infrared:
Part A: If the threshold wavelength is 900 nm, what is the work function of the photocathode? 1.38eV
Part B:If light of wavelength 640nm strikes such a photocathode, what will be the maximum kinetic energy, in eV, of the emitted electrons?
Explanation / Answer
a) so KEmax = hc/lambda - work
threshold when KE = 0
hc/lambda = work = 1240/900=1.38 eV
b) Kemax = hc/lambda - work = 1240/640 -1.38=0.558 eV