The effect of electric potential on the threshold frequency in the Lenard's photoelectric experiment

I have been taught that electrons can ejected from the atom by applying strong electric field(field emission),

and also by photoelectric effect even light of certain frequency for certain atom can cause ejection of electron. So combining these two

I was wondering if the electric field could decrease threshold frequency as electric field is already applying a force i.e less energy required as electric field is also causing ejection of electron simultaneously with the light ray

According to Davisson–Germer experiment on electron says they proved that wavelength inversely proportional to root of potential difference en.m.wikipedia.org/wiki/Davisson–Germer_experiment davisson germer

Is this true?Photoelectric affect effected by electric field.


1 Answer 1


No I don't think it is possible to do that because when a very strong electric field is applied the positive plate tries to attract the electrons very strongly and in the attempt rips them off their parent atom. On the other hand, when an electron absorbs a photon of suitable frequency, it can ionize the atom, leaving behind an ion and a free electron. You might be knowing that an electron will absorb a photon only if its wavelength matches the electron's or is an integral multiple of it. So suppose the electron is in the valence shell and it needs a photon of energy $h\nu$ to leave the atom. At the same time you apply a very high voltage across the plates,(a bit less than what is required to ionize the metal) and we incident a light of energy $h\nu_1$ ($\nu_1<\nu$), the metal atom won't be ionized as the electron won't even absorb this photon as it's wavelength isn't an integral multiple of that of the electron and the electric field also hasn't got enough strength to rip off the electron.

So, at the same time both photons and electric field can't be used to ionize an atom. Either of the two has to do the job and when ripping is done by electric field you will get a discharge tube or a cathode ray tube.

However you may use the electric field to increase the kinetic energy of the released electron to increase its probability of reaching the positive plate. This is generally done to increase the photo current to the saturation current.

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  • $\begingroup$ So applying electric field does not affect electrons wavelength ? $\endgroup$
    – user72730
    Mar 30, 2019 at 9:11
  • $\begingroup$ But Davisson–Germer experiment on electron says otherwise they proved that wavelength inversely proportional to root of potential difference ..en.m.wikipedia.org/wiki/Davisson–Germer_experiment $\endgroup$
    – user72730
    Mar 30, 2019 at 11:23
  • $\begingroup$ Yes, you are correct. However I didn't mean that. The wavelength changes when the electron is accelerated through V. But here you asked only about what happens when the electrons haven't yet been released from the metal. I mean you asked about how to ionize the atom with electric field. So as long as the electron hasn't been ripped off its atom its wavelength won't change. $\endgroup$ Mar 30, 2019 at 11:34

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