Modern Atomic Theory
Electromagnetic Radiation and the Idea of Quantum
The Photoelectric Effect Page [1 of 2]
In 1887 Hertz made an observation that again turned out to be crucial point in the development of quantum
Let me describe this experiment for you. This is referred to as the photoelectric effect.
What he observed
was that if you took a block of metal, let’s say potassium, and radiated it with light, that you could, in fact, kick out
electrons from that metal—eject electrons from that metal.
You could collect the electrons.
In fact, you actually could
fairly easily, with the instruments of the day, determine what the kinetic energies were of these electrons that were
being emitted from this metal.
Now, here’s the really amazing part.
As you adjust the properties of this light, you can adjust how these electrons are
being kicked out. But here’s the interesting thing.
If you increase the intensity of the light, you might think that the
electrons that are coming off will come off with a greater kinetic energy, and in fact, that turns out not to be true, that
adjusting intensity you get more electrons kicked off, but they don’t come of with any higher kinetic energy.
But if you
change the frequency of the light you’re radiating, that does correlate very strongly with the kinetic energy of the
electrons coming off.
Now, that’s peculiar.
Our intuitive feeling here is that the more intense the light is the more
energy there must be in that light, and so the more energy you must have in these electrons that you’re kicking out,
but not so. The frequency of the light ends up being much more the important player here in this kind of an
How could that be?
Let’s look at the data, again, what that would look like, because something else shows up very interesting.
this is a plot of the frequency of the light and the kinetic energy of the electrons that are being ejected.
here again this blue curve.
What you see is as we lower the frequency of light, no matter what the intensity is, we
lower the kinetic energy of the electrons that are being ejected.
Okay, good enough.
But if we drop the frequency too
far, we reach what’s called a “threshold frequency,” and below that threshold, no matter how intense the light is, we
don’t get a single electron emitted.
Something is very special about that frequency, below which,
again, we get no activity at all—no electrons are being kicked out.
So how do we explain this?