In the photocurrent experiment, if the intensity of light is kept constant, but the frequency is slightly increased, what's the effect on the saturation current?

I think saturation current should decrease because the number of photons should decrease.

But, all my reference books say that the saturation current depends only on intensity, and since it is constant, hence there will be no change.

Can you please tell me which reasoning is correct? And why?

EDIT: Another confusing factor is the energy of electrons. If the electrons have more energy due to the increased frequency then I would take less time to get to the anode and that should increase the current. Now, which factor is more dominant? And how do i know that?

  • $\begingroup$ clue: you need to consider number of photons (intensity) vs wavelength (frequency) and the work function. And what did you learn about the photo-electric effect? $\endgroup$
    – docscience
    Commented Apr 30, 2015 at 15:58
  • $\begingroup$ Please assume that the work function is low enough that emmission does always take place even after changing frequency a bit. @docscience i know the basic photoelectric equation by einstein and the rudimentary setup that goes along with it... $\endgroup$ Commented Apr 30, 2015 at 16:23

1 Answer 1


This is I guess a common problem that stems from the way we use the word "intensity". Normally, we would use intensity to mean something like the energy going through a unit area per unit time. In the case of the photoelectric effect, we instead generally mean the number of photons (per unit time), as it is this that decides how many photo-electrons are emitted, and directly determines the current.

Also note that there is no way we can have a "small" change in the number of photons, compared to unity. If you slightly increase the frequency of a monochromatic source (a phrase that typically means a source of fixed frequency) by an arbitrarily small amount, the energy-intensity will necessarily increase. Only when you get to the point where the increase in frequency can be compensated for by reducing the number of photons by exactly one is it possible to have the same energy-intensity as what you started out with. You can see that energy-intensity is somewhat more complicated to keep constant when you bring in photons.

Anyway, for such cases, all you have to do is look at the variation of saturation current with frequency for a fixed photon-number-intensity i.e. consider the frequency and photon number as independent variables.

EDIT: For the saturation current, we consider the case when all the electrons get there, and the number of these electrons (per unit time), which is the current, is given by the photon number. Making the electrons get there is the precise function of the anode voltage in these situations, and the saturation current appears with a very large voltage. Otherwise, the kinetic energies of the electrons is the basic reason behind the variation of current with the anode voltage.

  • $\begingroup$ Your last para is basically my question. Assuming i am able to keep energy intensity constant (which is quite tough), what can i conclude regarding variation of saturation current with frequency? Is it inversely or directly or not related? And please respond with a concrete answer. This is not a homework question, i am reading up as a hobby, so there will be no honor code violations etc.. $\endgroup$ Commented Apr 30, 2015 at 17:52
  • $\begingroup$ Your conclusions are ok for that case. $\endgroup$
    – AV23
    Commented Apr 30, 2015 at 17:56
  • $\begingroup$ Please look at the edit, @AV23 $\endgroup$ Commented Apr 30, 2015 at 18:01
  • $\begingroup$ So long story short, saturation current will be constant no matter what if the intensity of light is kept constant, right? $\endgroup$ Commented Apr 23, 2018 at 4:17

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