The Photoelectric Effect and Photon Energy

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The Photoelectric Effect and Photon Energy

Charge up a piece of zinc. (Make it negatively charged)

Shine electromagnetic (E-M) radiation on it starting with very long waves - e.g. radio waves. Nothing happens!

No matter how intense (bright) you make the waves... nothing!

Gradually shorten the wavelength of the E-M radiation and when you get to the U-V region, suddenly the zinc discharges. Even at a low intensity the effect will occur.

The Photoelectric Effect and Photon Energy Diagrams

There is a threshold frequency, fo, at which it starts to discharge.

What is going on?

Well, the E-M radiation is causing the electrons to leave the metal, making it discharge. We call these electrons photoelectrons. The idea of electromagnetic radiation making electrons leave isn't as crazy as it at first seems.


Because E-M waves deliver energy so if they deliver enough energy to a particular electron, surely that electron could use the energy to leave the metal surface and whiz off to freedom. So that's not a problem.

The problem is that low frequency (long wavelength) radiation won't do it but waves of higher frequency will. Wave theory says that any wave will deliver energy so surely if you shine any radiation onto the metal for long enough eventually enough energy will be delivered to allow electrons to leave. Also, if you increase the intensity of the radiation, the effect should occur sooner. But no!

The effect can only be explained if you realise that E-M radiation does not always behave like a wave - a smooth continuous stream of energy being delivered to a point. In this case you can only explain the photoelectric effect if E-M radiation is behaving like lumps or packets of energy being delivered one by one. We call these lumps quanta or photons.

Let's look at these photons. What determines the amount of energy in each photon?

Answer: The energy of a photon, E is given by

E = hf

where h = Planck's constant and f = the frequency of the radiation.

So the greater the frequency, the greater the energy.

The Photoelectric Effect and Photon Energy Diagrams

Note: E = hf can be written as Copyright S-cool as Copyright S-cool

So greater values of λ mean smaller values of energy.

The Photoelectric Effect and Photon Energy Diagrams

OK - so in E-M radiation of one frequency you have all the photons with the same energy. Billions of packets of energy being delivered every second.