# Photoelectric effect: a recreative quiz

Let us consider the photoelectric effect described on http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html/" [Broken].

A photon has an energy of 2.25 eV and produces a photoelectron effect on a potassium target,
the maximum energy of the photoelectron is 0.50 eV,
indicating the photoelectric threshold of potassium is 1.75 eV.

Calculating the momentum of the photon and phtoelectron gives:

momentum photon = 1.2E-27 N.s
momentum electron = 270E-27 N.s

The quiz is: what happened to the momentum conservation law?

Michel

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ZapperZ
Staff Emeritus
Let us consider the photoelectric effect described on http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html/" [Broken].

A photon has an energy of 2.25 eV,
a photoelectron is ejected from a potassium with the maximum energy of 0.50 eV,
indicating the photoelectric threshold of potassium is 1.75 eV.

Calculating the momentum of the photon and phtoelectron gives:

momentum photon = 1.2E-27 N.s
momentum electron = 270E-27 N.s

The quiz is: what happened to the momentum conservation law?

Michel

Nothing.

1. Photoemission theory has shown that the photon does not impart any significant transverse (in plane parallel to the surface) momentum to the photoelectron. The predominant origin of the transverse momentum is due to the transverse momentum of the electron while it is in the solid.

2. The photoelectron momentum in the perpendicular direction is due to a combination of (i) the momentum of the photon and (ii)the momentum of the electron while it is in the solid before it was liberated.

3. The lattice ions are necessary for the photoemission process because they take up the recoil momentum. This means that you cannot get the standard photoemission process from a free electron gas.

Zz.

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Too bad to ask such a quiz in an expert forum !
I should have tried an homework forum !

However, I think it is not necessary to make any reference to the internal structure of the target.
It is enough to recognize that the target plate can take off a recoil momentum and close the balance.

But your explanation is more physical !

Michel