# Photoelectric effect: according to classical physics?

This is a sample test question I've encountered twice in some practice chemistry finals. I'm a little bit confused about what it's asking.

You are conducting an experiment on the photoelectric effect, and observe that, at a certain frequency and intensity of light, no current flows. According to classical physics, what should you do to make current flow between the electrodes?

According to the Planck equation, $E=h\nu$, says that if I want electrons to escape from one electrode to the other, I need to hit it with a specific frequency. Is that correct? And is that the classical interpretation of the photoelectric effect? How would you phrase the answer to the question, in 3 sentences or fewer? Thanks!

According to the Planck equation, E=hν, says that if I want electrons to escape from one electrode to the other, I need to hit it with a specific frequency. Is that correct?

Yes.

And is that the classical interpretation of the photoelectric effect?

No. There is no classical interpretation of the photoelectric effect, that was why quantization became necessary.

In classical physics if no current flows the assumption is that the resistance is very high. The solution would be to raise the potential supplied to the electrodes.

One sees that this does not work. It works if the light has the specific frequency necessary for kicking out an electron, demonstrating that it is not a classical effect, and the voltage does need to be raised.

The student is being led to conclude himself ( herself) that one needs a quantum interpretation.

"According to classical physics, you should increase the potential between the electrodes to make the current flow." And as pointed out by Anna v there is no classical interpretation of Photoelectric effect. You may find this useful http://physics.info/photoelectric/

• "...classical physics would predict that a more intense beam of light would eject electrons with greater energy than a less intense beam no matter what the frequency." That's what I was looking for. Thanks again!
– Greg
Jul 10 '13 at 12:16

The 'classic' explanation means before the discovery of the photoelectric effect. It's a very poor question.

ps the Planck equation doesn't say anything about the photoelectric effect directly