# Problem: 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? Part B If light of wavelength 600 nm strikes such a photocathode, what will be the maximum kinetic energy, in , of the emitted electrons?

###### FREE Expert Solution

The work function:

$\overline{){\mathbf{\varphi }}{\mathbf{=}}{\mathbf{h}}{\mathbf{f}}}$, where f = c/λ

Part A

Φ = hc/λ

Φ = (6.63 × 10-34)(3.0 × 108)/(900 × 10-9) = 2.21 × 10-19

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###### Problem Details

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?

Part B If light of wavelength 600 nm strikes such a photocathode, what will be the maximum kinetic energy, in , of the emitted electrons?