Image intensifiers used in night-vision devices create a bright image from dim l

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 600 nm strikes such a photocathode, what will be the maximum kinetic energy, in eV, of the emitted electrons? K=?

Explanation / Answer

so for 900nm KE = 0
so work function = hc/=1240eVnm/900nm=1.378 eV

now we find max KE at 600 nm

KE = hc/ - work function = 1240/600-1.378=0.689 eV

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