Re: [Phys-L] Photoelectric effect: electron current vs. frequency

[John Denker <jsd@av8n.com>]

On 11/10/21 8:13 AM, Antti Savinainen via Phys-l wrote:

> For instance, the Khan Academy explains it thusly:
>
> "Because the light amplitude was kept constant as the light
> frequency increased, the/number/of photons being absorbed by the
> metal remained constant. Thus, the rate at which electrons were
> ejected from the metal (or the electric current) remained constant as
> well. The relationship between electron current and light frequency
> is illustrated in graph (b) above."
>
> However, when the situation is run using the Phet simulation, the 
> current doesn't stay constant: it increases when the frequency of 
> light increases (that is, the wavelength decreases).

Ugh.

> Which one is correct? 

Neither, although the latter is "less wrong".

It's a lot more complicated than that. There is a simple energy
argument: The photon needs to supply enough energy to pop the
electron out of the metal or semiconductor or gas molecule or
whatever. Differently-situated electrons have different amounts
of /initial/ energy (KE and PE), so different amounts of photon
energy are needed. As you increase the frequency, you can talk
to more and more electrons.

People have been doing this for about 70 years. Try googling for
photoemission spectroscopy.

https://www.nobelprize.org/uploads/2018/06/siegbahn-lecture-1.pdf

==========

There is a *threshold* frequency that is independent of intensity,
which can be found by curve fitting and extrapolating to zero ...
and the current is zero independent of frequency below threshold
... but that does not mean the current is independent of frequency
above threshold. Not independent, and also not a smoothly increasing
function.