# What does the photoelectric effect demonstrate

1. Apr 18, 2004

### Chen

In an exam today the teacher asked "What does the photoelectric effect demonstrate" and the answers were "The wave nature of light", "The particle nature of light", "The dual nature of light" (i.e both)" and "Neither".

I answered that it demonstrates the dual nature of light, but she insists that it is only the particle nature of it. I think she is wrong, because when analyzing the effect we talk about the photon's energy which is equal to $$h\nu$$, but $$\nu$$ is the frequency of light and that is clearly a wave property.

2. Apr 18, 2004

### SHM

Isn't it talking of light in terms of photons, which show a particle behaviour rather than a wavelike behaviour...

3. Apr 18, 2004

### jdavel

Chen,

Well, it wasn't a very fair question. If I had asked it on a test, I think I would have given credit for both answers.

That said, I think the pure particle answer is more right than the wave particle duality anwer. Frequency isn't just a wave property; it's a property of any vibration. So, the photoelectric effect could be explained with vibrating particles. It can't be explained with a wave.

4. Apr 18, 2004

### Chen

But we define the energy of each photon as $$h\nu$$ and that is a wave property. If I throw balls at a wall one a at a time I can't talk about their wave frequency, can I?

(Just saw your post) I agree that it's a "trick question". If the question was "Which behaviour of light does the effect prove", the answer would have been particle behaviour of course. But that wasn't the question...

5. Apr 18, 2004

### ZapperZ

Staff Emeritus
But that what makes it NOT a wave property. The energy of a classical wave is DEFINED as NOT being a function of frequency, but rather AMPLITUDE. The frequency only comes in as the "power" of the energy being transfered. The "frequency" involved in $$h\nu$$ involves the oscillation of the E-field within the "energy packet". This isn't a wave that most of us are familiar with.

So the photoelectric effect is one example of the discrete energy nature of EM radiation. So I would side with your teacher in this case.

Zz.

6. Apr 18, 2004

### meister

That's a bad question. Because in my mind, the explanation offered for the photoelectric effective effectively demonstrates the dual nature of light. Since light had already been thought of as a wave, when this experiment was explained by Einstein, it showed that light could be thought of as both a particle and a wave.

7. Apr 18, 2004

### JJ

What's the wave amplitude of a photon?

8. Apr 18, 2004

### Kurdt

Staff Emeritus
The photoelectric effect demonstrates the particle nature of light as it shows that light arrives in little packets called photons. in calssical wave nature the light could knock an electron out of orbit just by the accumulation of energy from one wave, but the photoelectric effect shows the light has to be above a certain frequency to free the electron thus dismissing the classical idea.

9. Apr 18, 2004

### Nereid

Staff Emeritus
Oh, tight spot!

Historically, the photoelectric effect played an important role in the debate about the wave vs particle nature of light, and this history is a ghost at our banquet (and is echoed in how teachers write exam questions). Also, ZapperZ well summarises a common view.

However, on balance I think Chen should have been sharp enough to intuit (?) what the teacher was looking for - after all, isn't the secret to getting good marks in a multiple-choice test to ask "now just what is the test-setter looking for here?"?

It reminds me of the student who was flunked for answering a question about using a barometer to measure the height of a very tall building (she answered, in effect, drop it from the top and measure how long it takes to smash into the footpath below). But that's for a different day ...

10. Apr 21, 2004

### NanoTech

Indeed, Einstien's Photoelectric Effect does show the possiblity of the dual nature of light. Most commonly observed phenomena with light can be explained by waves. But the photoelectric effect suggested a particle nature for light. This effect shows that light behaves as particles.I really think the answer depends on what level class you are taking. If it's an intro physics class, then I would have answered, "the particle nature of light."

Last edited: Apr 21, 2004