Image intensifiers used in night-vision devices create a bright image from dim l
ID: 1505199 • 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 um, in the infrared: If the threshold wavelength is 900 nm, what is the work function of the photocathode? Express your answer with the appropriate units. E= 1.38 eV If light of wavelength 660 nm 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
A) threshold wavelength = 900 nm
workfunction = hc/(threshold wavelength) = 6.26*10^-34*3*10^8/(900*10^-9) = 2.08*10^-19 J = 1.29 eV
B) hf = workfunction + Ek
=> Ek = [6.26*10^-34*3*10^8/(660*10^-9)] - 2.08*10^-19 = 0.76*10^-19 J = 0.474 eV