Image intensifiers used in night-vision devices create a bright image from dim l
ID: 1297500 • 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:
If light of wavelength 680nm strikes such a photocathode, what will be the maximum kinetic energy, in eV, of the emitted electrons?
Express your answer using two decimal places and include the appropriate units.
Explanation / Answer
KEmax = hc/lambda - work function
threshold when KE = 0
hc/lambda = work function = (6.625x10^-34*3x10^8)/(900x10^-9x1.6x10^-19) = 2.208 x 10^-19 J
we know ( 1 eV 1.6x10^-19 J )
work function = 1.38 eV
Kinetic Energy Maximum : -
Kmax = hc/lambda - work = (6.625x10^-34*3x10^8)/(680x10^-9x1.6x10^-19) -1.38= 1.8235 ev - 1.38 eV =
= 0.44 eV