How is photoelectron energy related to excitation wavelength?

How do you calculate the kinetic energy of a photoelectron produced in cesium by 400 nm light?

A photoelectron is an electron that is ejected from a substance (most often an alkali metal) after it absorbs electromagnetic radiation. In this case, shining violet light on cesium causes ejection of electrons. Some of the energy of the light goes into jarring the electron free of the metal; the rest appears as kinetic energy of the electron:
energy
per
photon
= energy to pull
one electron
out of metal
+ kinetic energy
per ejected
electron
h = h0 + h(-0)

where is the frequency of the incoming light, and 0 is the threshold frequency for the metal (cesium in your problem). You'll have to look up that threshold frequency to complete the problem.

For more detail on the photoelectric effect and its significance see the companion notes on the quantum theory.

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