Question

6. Think and discuss: When the photoelectric effect was discovered, scientists were surprised that low-frequency light was unable to remove electrons, even when emitted at extremely high intensities. (In other words, scientists expected the low frequency of the light to be offset by its high intensity.)

How does thinking about light as a stream of particles, rather than a single wave, explain this result? If possible, discuss your answer with your classmates and teacher.

Answer 1

Ah, so what does an electron do to a photon that doesn't have enough energy to "eject" the electron?

Answer 2

I thought "ionizing radiation," including UV light, was called this because indeed it does ionize, which is to remove one or more of the outer electrons from the affected atom.

Answer 3

Oh, I guess I did give a bit of a flase impression there.

You are absolutely correct. To observe the photoelectric effect, we put a voltage across a surface to lower the work function of the electrons. This makes lower energy photons "ionizing" in a sense, as they can kick out electrons from the material, but it isn't true ionizing radiation.

Thanks for catching that :)

Answer 4

Removed my first reply to avoid confusion. My original explanation went horribly awry.

For the photoelectric effect to occur, an electron must gain enough energy to escape the surface of the material (this energy is called the "work function"). Beyond that point, the voltage difference would carry the electron to the other end of the circuit.

If light were a continuous wave, electrons could gather this energy continuously, and then escape. But if light is a stream of particles, then a single particle needs to have all the energy for an electron to escape (otherwise the photon would just scatter). Since just a SINGLE particle needs to have the energy to accomplish this, putting more particles of low energy wouldn't have an effect.

So long as the photons energy is less than the work function, then it doesn't matter how many of those photons are around. Increasing the intensity is just increasing the number of low energy particles there are. But since these don't individually have enough energy to cause the photoelectric effect, we won't see anything happen in the experiment.